MR. JUSTICE BRENNAN delivered the opinion of the Court.[fn**] [fn**]446MR. JUSTICE STEWART and MR. JUSTICE POWELL join only Parts I, II-C, and III of this opinion. MR. JUSTICE WHITE, MR. JUSTICE MARSHALL, and MR. JUSTICE STEVENS join the entire opinion.

As part of a comprehensive program to recoup the costs of federal aviation programs from those who use the national airsystem, Congress in 1970 imposed an annual registration tax on all civil aircraft that fly in the navigable airspace of the United States. 26 U.S.C. § 4491. The constitutional question presented in this case is whether this tax, as applied to an aircraft owned by a State and used by it exclusively for police functions, violates the implied immunity of a state government from federal taxation. We hold that it does not.

I

Since the passage of the Air Commerce Act of 1926, 44 Stat. 568, the Federal Government has expended significant amounts of federal funds to develop and strengthen an integrated national airsystem and to make civil air transportation safe and practical. It has established, developed, and improved a wide array of air navigational facilities and services that benefit all aircraft flying in the Nation's navigable airspace, and it has also made substantial grants to state and local governments to assist in planning and developing airports.

In 1970, after an extended study of the national airsystem, Congress concluded that the level of annual federal outlays on aviation, while significant, had not been sufficient to permit the national airsystem to develop the capacity to cope satisfactorily with the current and projected growth in air transportation. To remedy this situation, Congress enacted two laws, the Airport and Airway Development Act of 1970 (Development Act), 84 Stat. 219, and the Airport and Airway Revenue Act of 1970 (Revenue Act), 84 Stat. 236, which together constitute a comprehensive program substantially to expand and improve the national airport and airway system over the decade beginning July 1, 1970. In the Development Act, Congress provided for vastly increased federal expenditures both for airport planning and development and for the further expansion of federal navigational services. More importantly for present purposes, the Revenue Act adopted several measures to ensure that federal outlays that benefited the civil users of the airways would, to a substantial extent, be financed by taxing measures imposed on those civil users. The Revenue Act, therefore, enacted for the first time, or increased, several taxes on civil aviation. Congress conceived of each of these revenue measures as user fees and calculated that they would produce revenues that would defray a significant and increasing percentage of the civil share of the annual total federal airport and airway expenditures for the fiscal years 1970 to 1979. To assure that the revenues from these user taxes would be expended only for the expansion, improvement, and maintenance of the air transportation system, an Airport and Airway Trust Fund was created, and Congress provided that the amount of revenue generated by the aviation user charges would, during the 1970's, be paid into this trust fund, as would any money appropriated from general revenues for aviation purposes. Revenue Act, § 208, 84 Stat. 250, 49 U.S.C. § 1742; see H.R. Rep. No. 91-601, p. 41 (1969) (hereinafter H.R. Rep.); S. Rep. No. 91-706, pp. 23-25 (1970) (hereinafter S. Rep.).

The financing measures in the Revenue Act are intended to promote two purposes. First, they are designed to serve the congressional policy of having those who especially benefit from Government activity help bear the cost. See H.R. Rep. 38; S. Rep. 5. Second, the financing provisions are intended to ensure that the capacity of the national air system would not again be found to be insufficient to meet the demands of increasing use. Congress believed that the inadequacy in past levels of investment in aviation had been due to the substantial competition from nonaviation budgetary requests. See H.R. Rep. 3. The trust fund and the user fees were, therefore, established to provide funding for aviation that would "generally match and grow with the demand" for use of the airways. Id., at 8.

The tax challenged in this case is one of several adopted in the Revenue Act, the annual aircraft registration tax. Revenue Act, § 206, 26 U.S.C. § 4491. It imposes an annual "flat fee" tax on all civil aircraft — including those owned by State and National Governments — that fly in the navigable airspace of the United States. The amount of the annual charge depends upon the type and weight of the aircraft: those with piston-driven engines pay $25 plus 2 cents per pound of the maximum certificated takeoff weight in excess of 2,500 pounds whereas turbine-powered aircraft pay $25 plus 3 1/2 cents per pound of the maximum certificated takeoff weight. See n. 1, supra.

As is apparent from both the rate of tax in § 4491 and the legislative history of the Revenue Act, Congress did not contemplate that the annual registration tax would generate significant amounts of revenue, but rather that the bulk of the funds generated by the system would come from other user taxes, each of which is related more directly to the level of use of the navigable airspace. Thus, commercial aviation's share of the cost of the federal activities would be raised primarily through an 8% tax on the price of domestic air passenger tickets, see Revenue Act, § 203, 26 U.S.C. § 4261; a $3 "head tax" on international flights originating in the United States, ibid.; and a 5% tax on the cost of transporting property by air, Revenue Act, § 204, 26 U.S.C. § 4271. Noncommercial general aviation — the generic category that includes state police aircraft — would pay most of its share through a 7-cent-per-gallon tax on aircraft fuel. See Revenue Act, § 202, 26 U.S.C. § 4041.

But while the registration tax was expected to produce only modest revenues and was understood to be only indirectly related to system use, Congress regarded it as an integral and essential part of the network of user charges. Moreover, it is the only tax imposed on those general noncommercial aircraft owned and operated by States. Although Congress was generally of the view that the States should be required to pay aviation user charges since "there would appear to be no reason why [they] should not pay for their fair share of the use of the airway facilities," H.R. Rep. 46; see S. Rep. 17-18, and in fact made the States subject to all the other user charges, it retained a statutory exemption for the States from the aircraft fuel, tire, and tube taxes. See 68A Stat. 480, as amended, 26 U.S.C. § 4041 (g) (1976 ed.); 26 U.S.C. § 4221.

The Commonwealth of Massachusetts owns several aircraft that are subject to the tax imposed by § 4491, including a helicopter which the Commonwealth uses exclusively for patrolling highways and other police functions. In 1973 the United States notified the Commonwealth that it had been assessed for a tax of $131.43 on this state police helicopter for the period from July 1, 1970, to June 30, 1971. The Commonwealth refused to pay and the United States thereafter levied on one of the Commonwealth's bank accounts and collected this tax, plus interest and penalties.

Pursuant to 28 U.S.C. § 1346 (1970 ed. and Supp. V), the Commonwealth then instituted this action for a refund of the money collected, contending that the United States may not constitutionally impose a tax that directly affects the essential and traditional state function of operating a police force. The District Court dismissed the complaint in an unreported decision. It first indicated its view that the most recent decisions of this Court had so limited a State's constitutional immunity from federal taxation that a constitutional challenge could not succeed unless the tax was discriminatory or the State showed that the tax actually impaired a State function. Because the Commonwealth had not alleged that this nondiscriminatory annual fee had in fact impaired the operations of its police force, the District Court concluded dismissal was mandatory. In the alternative, the District Court held that the tax in question is a user fee and that, whatever the present scope of the constitutional principle of implied immunity of a state government from federal taxes, a user fee does not implicate the doctrine. The Court of Appeals for the First Circuit affirmed, solely on the latter ground. 548 F.2d 33 (1977). We granted certiorari, 432 U.S. 905 (1977), to resolve a conflict between this decision and Georgia Dept. of Transp. v. United States, 430 F. Supp. 823 (ND Ga. 1976), appeal docketed, No. 77-16. See also City of New York v. United States, 394 F. Supp. 641 (SDNY 1975), affirmance order, 538 F.2d 308 (CA2 1976); Texas v. United States, 72-2 USTC ¶ 16.048 (WD Tex. 1972), aff'd, 73-1 USTC ¶ 16,085 (CA5 1973) (holding that 8% air passenger tax may constitutionally be applied to state employees traveling on official state business). We affirm.

II

A review of the development of the constitutional doctrine of state immunity from federal taxation is a necessary preface to decision of this case. For while the Commonwealth concedes that certain types of user fees may constitutionally be applied to its essential activities, it urges that the decisions of this Court teach that the validity of any impost levied against a State must be judged by a "bright-line" test: If the measure is labeled a tax and/or imposed or collected pursuant to the Internal Revenue Code, it is unconstitutional as applied to an essential state function even if the revenue measure operates as a user fee. See Brief for Petitioner 14-28. And the Commonwealth maintains that § 4491 is invalid for the additional reason that the values furthered by this constitutional doctrine necessarily require the invalidation of a levy such as that under § 4491 which, as an annual fee, is not directly related to use. See Brief for Petitioner 28-41. Neither contention has merit. The principles that have animated the development of the doctrine of state tax immunity and the decisions of this Court in analogous contexts persuade us that a State enjoys no constitutional immunity from a nondiscriminatory revenue measure, like § 4491, which operates only to ensure that each member of a class of special beneficiaries of a federal program pay a reasonable approximation of its fair share of the cost of the program to the National Government. Like the Court of Appeals, we have no occasion to decide either the present vitality of the doctrine of state tax immunity or the conditions under which it might be invoked.

A

That the existence of the States implies some restriction on the national taxing power was first decided in Collector v. Day, 11 wall. 113 (1871). There this Court held that the immunity that federal instrumentalities and employees then enjoyed from state taxation, see Dobbins v. Commissioners, 16 Pet. 435 (1842); McCulloch v. Maryland, 4 Wheat. 316 (1819), was to some extent reciprocal and that the salaries paid state judges were immune from a nondiscriminatory federal tax. This immunity of State and Federal Governments from taxation by each other was expanded in decisions over the last third of the 19th century and the first third of this century, see, e.g., Panhandle Oil Co. v. Mississippi ex rel. Knox, 277 U.S. 218 (1928); Indian Motorcycle Co. v. United States, 283 U.S. 570 (1931) (sales from a private person to one sovereign may not be taxed by the other), but more recent decisions of this Court have confined the scope of the doctrine.

The immunity of the Federal Government from state taxation is bottomed on the Supremacy Clause, but the States' immunity from federal taxes was judicially implied from the States' role in the constitutional scheme. Collector v. Day, supra, emphasized that the States had been in existence as independent sovereigns when the Constitution was adopted, and that the Constitution presupposes and guarantees the continued existence of the States as governmental bodies performing traditional sovereign functions. 11 Wall., at 125-126. To implement this aspect of the constitutional plan, Collector v. Day concluded that it was imperative absolutely to prohibit any federal taxation that directly affected a traditional state function, quoting Mr. Chief Justice Marshall's aphorisms that "`the power of taxing . . . may be exercised so far as to destroy,'" id., at 123, quoting McCulloch v. Maryland, supra, at 427, and "`a right [to tax], in its nature, acknowledges no limits.'" 11 Wall., at 123, quoting Weston v. Charleston, 2 Pet. 449, 466 (1829). The Court has more recently remarked that these maxims refer primarily to two attributes of the taxing power. First, in imposing a tax to support the services a government provides to the public at large, a legislature need not consider the value of particular benefits to a taxpayer, but may assess the tax solely on the basis of taxpayers' ability to pay. Second (of perhaps greater concern in the present context), a tax is a powerful regulatory device; a legislature can discourage or eliminate a particular activity that is within its regulatory jurisdiction simply by imposing a heavy tax on its exercise. See National Cable Television Assn. v. United States, 415 U.S. 336, 340-341 (1974). Collector v. Day, like the earlier McCulloch v. Maryland, reflected the view that the awesomeness of the taxing power required a flat and absolute prohibition against a tax implicating an essential state function because the ability of the federal courts to determine whether particular revenue measures would or would not destroy such an essential function was to be doubted.

As the contours of the principle evolved in later decisions, "cogent reasons" were recognized for narrowly limiting the immunity of the States from federal imposts. See Helvering v. Gerhardt, 304 U.S. 405, 416 (1938). The first is that any immunity for the protection of state sovereignty is at the expense of the sovereign power of the National Government to tax. Therefore, when the scope of the States' constitutional immunity is enlarged beyond that necessary to protect the continued ability of the States to deliver traditional governmental services, the burden of the immunity is thrown upon the National Government without any corresponding promotion of the constitutionally protected values. See, id., at 416-417; Helvering v. Mountain Producers Corp., 303 U.S. 376, 384-385 (1938); Willcuts v. Bunn, 282 U.S. 216, 225 (1931). The second, also recognized by Mr. Chief Justice Marshall in McCulloch v. Maryland, supra, at 435-436, is that the political process is uniquely adapted to accommodating the competing demands "for national revenue, on the one hand, and for reasonable scope for the independence of state action, on the other," Helvering v. Gerhardt, supra, at 416: The Congress, composed as it is of members chosen by state constituencies, constitutes an inherent check against the possibility of abusive taxing of the States by the National Government.

In tacit, and at times explicit, recognition of these considerations, decisions of the Court either have declined to enlarge the scope of state immunity or have in fact restricted its reach. Typical of this trend are decisions holding that the National Government may tax revenue-generating activities of the States that are of the same nature as those traditionally engaged in by private persons. See, e.g., New York v. United States, 326 U.S. 572 (1946) (tax on water bottled and sold by State upheld); Allen v. Regents, 304 U.S. 439 (1938) (tax on admissions to state athletic events approved notwithstanding use of proceeds for essential state functions); Helvering v. Powers, 293 U.S. 214 (1934) (tax on operations of railroad by State); Ohio v. Helvering, 292 U.S. 360 (1934) (tax on state liquor operation); South Carolina v. United States, 199 U.S. 437 (1905) (tax on state-run liquor business). It is true that some of the opinions speak of the state activity taxed as "proprietary" and thus not an immune essential governmental activity, but the opinions of the Members of the Court in New York v. United States, supra, the most recent decision, rejected the governmental-proprietary distinction as untenable. Rather the majority reasoned that a nondiscriminatory tax may be applied to a state business activity where, as was the case there, the recognition of immunity would "accomplish a withdrawal from the taxing power of the nation a subject of taxation of a nature which has been traditionally within that power from the beginning. Its exercise . . . by a nondiscriminatory tax, does not curtail the business of the state government more than it does the like business of the citizen." 326 U.S., at 588-589 (Stone, C.J., concurring).

Illustrative of decisions actually restricting the scope of the immunity is the line of cases that culminated in the overruling of Collector v. Day in Graves v. New York ex rel. O'Keefe, 306 U.S. 466 (1939). See, e.g., Helvering v. Gerhardt, supra; Helvering v. Mountain Producers Corp., supra; Metcalf Eddy v. Mitchell, 269 U.S. 514 (1926). Collector v. Day, of course, involved a nondiscriminatory tax that was imposed not directly on the State but rather on the salary earned by a judicial officer. Neither Collector v. Day itself nor its progeny or precursors made clear how such a taxing measure could be employed to preclude the States from performing essential functions. In any case, in the line of decisions that culminated in Graves v. New York ex rel. O'Keefe, supra, the Court demonstrated that an immunity for the salaries paid key state officials is not justifiable. Although key state officials are agents of the State, they are also citizens of the United States, so their income is a natural subject for income taxation. See Helvering v. Gerhardt, supra, at 420 and 422.

More significantly, because the taxes imposed were nondiscriminatory and thus also applicable to income earned by persons in private employment, the risk was virtually nonexistent that such revenue provisions could significantly impede a State's ability to hire able persons to perform its essential functions. See Graves v. New York ex rel. O'Keefe, supra, at 484-485; Helvering v. Gerhardt, supra, at 420-421. The only advantage conceivably to be lost by denying the States such an immunity is that essential state functions might be obtained at a lesser cost because employees exempt from taxation might be willing to work for smaller salaries. See 304 U.S., at 420-421. But that was regarded as an inadequate ground for sustaining the immunity and preventing the National Government from requiring these citizens to support its activities. See Graves v. New York ex rel. O'Keefe, supra, at 483 and cases cited in n. 3. The purpose of the implied constitutional restriction on the national taxing power is not to give an advantage to the States by enabling them to engage employees at a lower charge than those paid by private entities, see Helvering v. Gerhardt, supra, at 421-422, but rather is solely to protect the States from undue interference with their traditional governmental functions. While a tax on the salary paid key state officers may increase the cost of government, it will no more preclude the States from performing traditional functions than it will prevent private entities from performing their missions. See Graves v. New York ex rel. O'Keefe, supra, at 484-485; Helvering v. Gerhardt, supra, at 420-421.

These two lines of decisions illustrate the "practical construction" that the Court now gives the limitation the existence of the States constitutionally imposes on the national taxing power; "that limitation cannot be so varied or extended as seriously to impair either the taxing power of the government imposing the tax . . . or the appropriate exercise of the functions of the government affected by it." New York v. United States, 326 U.S., at 589-590 (Stone, C.J., concurring) quoting Metcalf Eddy v. Mitchell, supra, at 523-524. Where the subject of tax is a natural and traditional source of federal revenue and where it is inconceivable that such a revenue measure could ever operate to preclude traditional state activities, the tax is valid. While the Court has by no means abandoned its doubts concerning its ability to make particularized assessments of the impact of revenue measures on essential state operations, compare New York v. United States, supra, at 581 (opinion of Frankfurter, J.) with 326 U.S., at 590 (Stone, C.J., concurring), it has recognized that some generic types of revenue measures could never seriously threaten the continued functioning of the States and hence are outside the scope of the implied tax immunity.

B

A nondiscriminatory taxing measure that operates to defray the cost of a federal program by recovering a fair approximation of each beneficiary's share of the cost is surely no more offensive to the constitutional scheme than is either a tax on the income earned by state employees or a tax on a State's sale of bottled water. The National Government's interest in being compensated for its expenditures is only too apparent. More significantly perhaps, such revenue measures by their very nature cannot possess the attributes that led Mr. Chief Justice Marshall to proclaim that the power to tax is the power to destroy. There is no danger that such measures will not be based on benefits conferred or that they will function as regulatory devices unduly burdening essential state activities. It is, of course, the case that a revenue provision that forces a State to pay its own way when performing an essential function will increase the cost of the state activity. But Graves v. New York ex rel. O'Keefe, and its precursors, see 306 U.S., at 483 and the cases cited in n. 3, teach that an economic burden on traditional state functions without more is not a sufficient basis for sustaining a claim of immunity. Indeed, since the Constitution explicitly requires States to bear similar economic burdens when engaged in essential operations, see U.S. Const., Amdts. 5, 14; Pennsylvania Coal Co. v. Mahon, 260 U.S. 393 (1922) (State must pay just compensation when it "takes" private property for a public purpose); U.S. Const., Art. I, § 10, cl. 1; United States Trust Co. v. New Jersey, 431 U.S. 1 (1977) (even when burdensome, a State often must comply with the obligations of its contracts), it cannot be seriously contended that federal exactions from the States of their fair share of the cost of specific benefits they receive from federal programs offend the constitutional scheme.

Our decisions in analogous context support this conclusion. We have repeatedly held that the Federal Government may impose appropriate conditions on the use of federal property or privileges and may require that state instrumentalities comply with conditions that are reasonably related to the federal interest in particular national projects or programs. See, e.g., Ivanhoe Irrigation Dist. v. McCracken, 357 U.S. 275, 294-296 (1958); Oklahoma v. Civil Service Comm'n, 330 U.S. 127, 142-144 (1947); United States v. San Francisco, 310 U.S. 16 (1940); cf. National League of Cities v. Usery, 426 U.S. 833, 853 (1976); Fry v. United States, 421 U.S. 542 (1975). A requirement that States, like all other users, pay a portion of the costs of the benefits they enjoy from federal programs is surely permissible since it is closely related to the federal interest in recovering costs from those who benefit and since it effects no greater interference with state sovereignty than do the restrictions which this Court has approved.

A clearly analogous line of decisions is that interpreting provisions in the Constitution that also place limitations on the taxing power of government. See, e.g., U.S. Const., Art. I, § 8, cl. 3 (restricting power of States to tax interstate commerce); § 10, cl. 3 (prohibiting any state tax that operates "to impose a charge for the privilege of entering, trading in, or lying in a port." Clyde Mallory Lines v. Alabama ex rel. State Docks Comm'n, 296 U.S. 261, 265-266 (1935)). These restrictions, like the implied state tax immunity, exist to protect constitutionally valued activity from the undue and perhaps destructive interference that could result from certain taxing measures. The restriction implicit in the Commerce Clause is designed to prohibit States from burdening the free flow of commerce, see generally Complete Auto Transit, Inc. v. Brady, 430 U.S. 274 (1977), whereas the prohibition against duties on the privilege of entering ports is intended specifically to guard against local hindrances to trade and commerce by vessels. See Packet Co. v. Keokuk, 95 U.S. 80, 85 (1877).

Our decisions implementing these constitutional provisions have consistently recognized that the interests protected by these Clauses are not offended by revenue measures that operate only to compensate a government for benefits supplied. See, e.g., Clyde Mallory Lines v. Alabama, supra (flat fee charged each vessel entering port upheld because charge operated to defray cost of harbor policing); Evansville-Vanderburgh Airport Authority v. Delta Airlines, Inc., 405 U.S. 707 (1972) ($1 head tax on explaining commercial air passengers upheld under the Commerce Clause because designed to recoup cost of airport facilities). A governmental body has an obvious interest in making those who specifically benefit from its services pay the cost and, provided that the charge is structured to compensate the government for the benefit conferred, there can be no danger of the kind of interference with constitutionally valued activity that the Clauses were designed to prohibit.

C

Having established that taxes that operate as user fees may constitutionally be applied to the States, we turn to consider the Commonwealth's argument that § 4491 should not be treated as a user fee because the amount of the tax is a flat annual fee and hence is not directly related to the degree of use of the airways. This argument has been confronted and rejected in analogous contexts. Capitol Greyhound Lines v. Brice, 339 U.S. 542 (1950), is illustrative. There the Court rejected an attack under the Commerce Clause on an annual Maryland highway tax of "2% upon the fair market value of motor vehicles used in interstate commerce." The carrier argued that the correlation between the tax and use was not sufficiently precise to sustain the tax as a valid user charge. Noting that the tax "should be judged by its result, not its formula, and must stand unless proven to be unreasonable in amount for the privilege granted," id., at 545, the Court rejected the carrier's argument:

"Complete fairness would require that a state tax formula vary with every factor affecting appropriate compensation for road use. These factors, like those relevant in considering the constitutionality of other state taxes, are so countless that we must be content with `rough approximation rather than precision.' . . . Each additional factor adds to administrative burdens of enforcement, which fall alike on taxpayers and government. We have recognized that such burdens may be sufficient to justify states in ignoring even such a key factor as mileage, although the result may be a tax which on its face appears to bear with unequal weight upon different carriers. . . . Upon this type of reasoning rests our general rule that taxes like that of Maryland here are valid unless the amount is shown to be in excess of fair compensation for the privilege of using state roads." Id., at 546-547. (Citations and footnotes omitted.)

See also Aero Mayflower Transit Co. v. Board of Railroad Comm'rs, 332 U.S. 495 (1947) (taxes of $10 and $15 per vehicle sustained against Commerce Clause challenges); Clyde Mallory Lines v. Alabama ex rel. State Docks Comm'n, supra (flat fee designed to defray cost of policing port upheld against claim it was constitutionally prohibited tax on privilege of entering harbor). This Court recently relied upon this reasoning to uphold a tax on commercial aviation activity. In Evansville-Vanderburgh Airport Authority v. Delta Airlines, Inc., supra, we sustained against claims based on the Commerce Clause and on the right to travel a $1 head tax on commercial airline passengers. We held that such taxes are valid so long as they (1) do not discriminate against interstate commerce, (2) are based upon some fair approximation of use, and (3) are not shown to be excessive in relation to the cost to the government of the benefits conferred. 405 U.S., at 716-720.

The Commonwealth, of course, recognizes that flat fees, and even flat annual fees, have been held constitutionally permissible in these contexts. It urges, however, that such "rough approximations of cost," while appropriate compensatory measures in other settings, should not be permissible here. It maintains that the values protected by the doctrine of state tax immunity require that any user tax be closely calibrated to the amount of any taxpayer's actual use, and it suggests that we — for purposes of the state tax immunity doctrine only — define user fees as charges for measurable amounts of use of government facilities.

We note first that it is doubtful that the National Government could recover the costs of its aviation activities from those direct beneficiaries without making at least some use of annual flat fees. In arguing that the Revenue Act provisions are not sufficiently user related, the Commonwealth places extensive reliance upon the DOT Study, prepared at the direction of Congress, of the best way to recoup the costs of the federal aviation activities from its beneficiaries. While the report recognized that it would be generally possible, albeit costly in the case of general aviation, to tie the charges to specific measurable benefits received, see DOT Study 61, it indicated that certain costs imposed by general aviation could only be recovered through flat fees. Id., at 61 n. 2.

But even if it were feasible to recover all costs through charges for measurable amounts of use of Government facilities, we fail to see how such a requirement would appreciably advance the policies embodied in the doctrine of state tax immunity. Since a State has no constitutional complaint when it is required to pay the cost of benefits received, the Commonwealth's only legitimate fear is that the flat-fee requirement may result in the collection from it of more than its actual "fair share." We observe first that where the charges imposed by the Federal Government apply to large numbers of private parties as well as to state activities, it is as likely as not that the user fee will result in exacting less money from the State than it would have to pay under a perfect user-fee system. More fundamentally, even when an annual flat fee results in some overcharges, the Commonwealth's solution would often increase the fiscal burden on the States. If the National Government were required more precisely to calibrate the amount of the fee to the extent of the actual use of the airways, administrative costs would increase and so would the amount of revenue needed to operate the system. The resulting increment in a State's actual fair share might well be greater than any overcharge resulting from the present fee system. But the complete answer to the Commonwealth's concern is that even if the flat fee does cost it somewhat more than it would have to pay under a perfect user-fee system, there is still no interference with the values protected by the implied constitutional tax immunity of the States. The possibility of a slight overcharge is no more offensive to the constitutional structure than is the increase in the cost of essential operations that results either from the fact that those who deal with the State may be required to pay nondiscriminatory taxes on the money they receive or from the fact a jury may award an eminent domain claimant an amount in excess of what would be "just compensation" in an ideal system of justice.

Whatever the present scope of the principle of state tax immunity, a State can have no constitutional objection to a revenue measure that satisfies the three-prong test of Evansville-Vanderburgh Airport Authority v. Delta Airlines, Inc. — substituting "state function" for "interstate commerce" in that test. So long as the charges do not discriminate against state functions, are based on a fair approximation of use of the system, and are structured to produce revenues that will not exceed the total cost to the Federal Government of the benefits to be supplied, there can be no substantial basis for a claim that the National Government will be using its taxing powers to control, unduly interfere with, or destroy a State's ability to perform essential services. The requirement that total revenues not exceed expenditures places a natural ceiling on the total amount that such charges may generate and the further requirement that the measure be reasonable and nondiscriminatory precludes the adoption of a charge that will unduly burden state activities.

III

Applying these principles to this case demonstrates that the Commonwealth's claim of constitutional immunity is particularly insubstantial. First, there is no question but that the tax imposed by § 4491 is nondiscriminatory. It applies not only to private users of the airways but also to civil aircraft operated by the United States — facts which minimize, if not eliminate entirely, the basis for a conclusion that § 4491 might be an abusive exercise of the taxing power. Indeed, the Revenue Act discriminates in favor of the States since it retains the States' exemption from the 7-cent-per-gallon fuel tax that applies to private noncommercial general aviation — a fact that illustrates the manner in which the political process is peculiarly adapted to the protection of state interests.

Second, the tax satisfies the requirement that it be a fair approximation of the cost of the benefits civil aircraft receive from the federal activities. As we have indicated, the legislative background and terms of the Revenue Act indicate that Congress believed that four measures, taken together, would fairly reflect some of the cost of the benefits that redound to the noncommercial general aircraft that fly in the navigable airspace of the United States: a 7-cent-per-gallon fuel tax, a 5-cent-per-pound tax on aircraft tires, a 10-cent-per-pound tax on tubes, see 26 U.S.C. § 4071, and the annual aircraft registration tax. See nn. 4 and 8, supra. The formula contained in these four measures taken together does not, of course, give weight to every factor affecting appropriate compensation for airport and airway use. A probable deficiency in the formula arises because not all aircraft make equal use of the federal navigational facilities or of the airports that have been planned or constructed with federal assistance. But the present scheme nevertheless is a fair approximation of the cost of the benefits each aircraft receives. Every aircraft that flies in the navigable airspace of the United States has available to it the navigational assistance and other special services supplied by the United States. And even those aircraft, if there are any, that have never received specific services from the National Government benefit from them in the sense that the services are available for their use if needed and in that the provision of the services makes the airways safer for all users. The four taxes, taken together, fairly reflect the benefits received, since three are geared directly to use, whereas the fourth, the aircraft registration tax, is designed to give weight to factors affecting the level of use of the navigational facilities. See n. 9, supra. A more precisely calibrated formula — which would include landing fees, charges for specific services received, and less reliance on annual flat fees, see DOT Study 62 — would, of course, be administratively more costly.

It follows that a State may not complain of the application of § 4491 on the ground it is not a fair approximation of use. Since the fuel tax, tire and tube tax, and annual registration fee together constitute an appropriate means of recovering the amount of the federal investment, a State, being exempt from the fuel, tire and tube taxes, can have no constitutional objection to the application of the registration fee alone.

Finally, the tax is not excessive in relation to the cost of the Government benefits supplied. When Congress enacted the Revenue Act, it contemplated that the user fees imposed on civil aircraft would not be sufficient to cover the federal expenditures on civil aviation in any one year, see n. 4, supra, and the actual experience during the first years of operation was that the revenues fell far short of covering the annual civil aviation outlays. Since the Commonwealth pays far less than private noncommercial users of the airways, there therefore is no basis for a conclusion that the application of the registration tax to the States produces revenues in excess of the costs incurred by the Federal Government. Affirmed.

MR. JUSTICE BLACKMUN took no part in the consideration or decision of this case.