opinion: Per Curiam.
  These cases arise under the Clean Air Act, as amended by the Clean Air Amendments of 1970, 84 Stat. 1676, 42 U. S. C. § 1857 et seg., and raise questions concerning the authority of the Administrator of the Environmental Protection Agency to compel various types of implementation and enforcement actions by the States. Four separate decisions in the Courts of Appeals reviewed transportation control plans promulgated by the Administrator for several States which had previously failed to submit adequate plans of their own. Four petitions have been filed seeking review of those decisions which, with limited exceptions, invalidated the Administrator’s transportation control plans which had been adopted in the form of regulations.
  Those transportation control plans have a variety of aspects which need not be discussed in great detail to explain our disposition of these cases. In general, they imposed upon the States the obligations (1) to develop an inspection and maintenance program pertaining to the vehicles registered in the affected Air Quality Control Regions, and to submit to the Administrator, by fixed deadlines, both a schedule of compliance and the operative regulations by which the program was to be run; (2) to develop various retrofit programs pertaining to several classes of older vehicles, in order to minimize several different types of emissions; (3) to designate and enforce preferential bus and carpool lanes, on streets sometimes specifically identified in the regulations and sometimes left to be chosen by the State; (4) to develop a program to monitor actual emissions as affected by the foregoing programs; and (5) to adopt certain other programs which varied from State to State.
  The critical fact about all of the foregoing obligations was that they were imposed on the States, under 40 CFR § 52.23 (1976), as elements of an applicable implementation plan. A State's failure to carry out any of them would therefore not merely allow the Administrator to step in and carry them out himself under § 113 (a) (2) of the Clean Air Act, but would, in the view of each of the Courts of Appeals, render the State “in violation of any requirement of an applicable implementation plan” and therefore apparently subject to direct enforcement actions against it under the provisions of § 113 (a)(1), 42 U. S. C. § 1857c~8 (a)(1):
  “Whenever, on the basis of any information available to him, the Administrator finds that any person is in violation of any requirement of an applicable implementation plan, the Administrator shall notify the person in violation of the plan and the State in which the plan applies of such finding. If such violation extends beyond the 30th day after the date of the Administrator's notification, the Administrator may issue an order requiring such person to comply with the requirements of such plan or he may bring a civil action in accordance with subsection (b) of this section.”
  Under dual challenges by the States that these regulations were not within the mandate of the Act, and that if they were they were in violation of the Constitution, the United States Courts of Appeals for the Ninth, Fourth, and District of Columbia Circuits struck them down. All of the courts rested on statutory interpretation, but noted also that serious constitutional questions might be raised if the statute were read as the United States argued it should be. Brown v. EPA, 521 F. 2d 827 (CA9 1975); Arizona v. EPA, 521 F. 2d 825 (CA9 1975); District of Columbia v. Train, 172 U. S. App. D. C. 311, 521 F. 2d 971 (1975); Maryland v. EPA, 530 F. 2d 215 (CA4 1975). The only substantial variation in the outcome of these decisions was that the District of Columbia Circuit affirmed regulations requiring the creation of bus lanes, the purchase by the affected jurisdictions of a fixed number of new buses, and the denial of registration to a vehicle whose owner is unable to produce a federal certificate of compliance, should a federal inspection program be instituted.
  The Solicitor General’s petitions from all three Courts of Appeals challenged them only insofar as they invalidated the regulations requiring state inspection and maintenance programs. In addition, we granted the petition for certiorari of the Commonwealth of Virginia on its challenge to the regulations which the District of Columbia Circuit had upheld. Prior to argument, the Solicitor General informed the Court that repeal of the bus purchase regulations was imminent, Reply Brief for Federal Parties 25, and that issue was thereby effectively removed from the case. Thus the litigation has undergone a great deal of shrinkage since the decisions below due to the federal parties’ exercise of their prerogative not to seek review of the invalidation of certain regulations.
  But the federal parties have not merely renounced an intent to pursue certain specified regulations; they now appear to admit that those remaining in controversy are invalid unless modified in certain respects:
  “The Administrator . . . concedes the necessity of removing from the regulations all requirements that the States submit legally adopted regulations; the [Administrator’s] regulations contain no requirement that the State adopt laws.” Brief for Federal Parties 20 n. 14.
  The federal parties’ position now appears to be that, while the challenged transportation plans do not require the enactment of state legislation, they do now contain, and must be modified to eliminate, certain requirements that the State promulgate regulations. See Reply Brief for Federal Parties 14 n. 22.
  We decline the federal parties’ invitation to pass upon the EPA regulations, when the only ones before us are admitted to be in need of certain essential modifications. Such action on our part would amount to the rendering of an advisory opinion. For this Court to review regulations normally required to be first reviewed in the Court of Appeals, before such review is had, is extraordinary. See Youngstown Sheet & Tube Co. v. Sawyer, 343 U. S. 579, 584-585 (1952). For it to review regulations not yet promulgated, the final form of which has only been hinted at, would be wholly novel. See generally Columbia Broadcasting System v. United States, 316 U. S. 407, 417-419 (1942); United States v. Los Angeles & Salt Lake R. Co., 273 U. S. 299, 309-310 (1927).
  The judgments of the respective Courts of Appeals are vacated, and the cases are remanded for consideration of mootness and such other proceedings as may be consistent with this opinion.
  It is so ordered.
  Section 113 (a)(2), 42 U. S. C. § 1857c-8 (a)(2), provides:
  “Whenever, on the basis of information available to him, the Administrator finds that violations of an applicable implementation plan are so widespread that such violations appear to result from a failure of the State in which the plan applies to enforce the plan effectively, he shall so notify the State. If the Administrator finds such failure extends beyond the 30th day after such notice, he shall give public notice of such finding. During the period beginning with such public notice and ending when such State satisfies the Administrator that it will enforce such plan (hereafter referred to in this section as ‘period of federally assumed enforcement’), the Administrator may enforce any requirement of such plan with respect to any person—
  “(A) by issuing an order to comply with such requirement, or
  “(B) by bringing a civil action under subsection (b) of this section.”
  Prior to the decision of the Ninth Circuit, a similar set of regulations pertaining to Pennsylvania had been upheld by the Third Circuit. Pennsylvania v. EPA, 500 F. 2d 246 (1974). That decision is not presently before the Court.
  The regulations were officially rescinded on February 8, 1977. 42 Fed. Reg. 7957.
choices: comity: civil rights comity: criminal procedure comity: First Amendment comity: habeas corpus comity: military comity: obscenity comity: privacy comity: miscellaneous comity primarily removal cases, civil procedure (cf. comity, criminal and First Amendment); deference to foreign judicial tribunals assessment of costs or damages: as part of a court order Federal Rules of Civil Procedure including Supreme Court Rules, application of the Federal Rules of Evidence, Federal Rules of Appellate Procedure in civil litigation, Circuit Court Rules, and state rules and admiralty rules judicial review of administrative agency's or administrative official's actions and procedures mootness (cf. standing to sue: live dispute) venue no merits: writ improvidently granted no merits: dismissed or affirmed for want of a substantial or properly presented federal question, or a nonsuit no merits: dismissed or affirmed for want of jurisdiction (cf. judicial administration: Supreme Court jurisdiction or authority on appeal from federal district courts or courts of appeals) no merits: adequate non-federal grounds for decision no merits: remand to determine basis of state or federal court decision (cf. judicial administration: state law) no merits: miscellaneous standing to sue: adversary parties standing to sue: direct injury standing to sue: legal injury standing to sue: personal injury standing to sue: justiciable question standing to sue: live dispute standing to sue: parens patriae standing standing to sue: statutory standing standing to sue: private or implied cause of action standing to sue: taxpayer's suit standing to sue: miscellaneous judicial administration: jurisdiction or authority of federal district courts or territorial courts judicial administration: jurisdiction or authority of federal courts of appeals judicial administration: Supreme Court jurisdiction or authority on appeal or writ of error, from federal district courts or courts of appeals (cf. 753) judicial administration: Supreme Court jurisdiction or authority on appeal or writ of error, from highest state court judicial administration: jurisdiction or authority of the Court of Claims judicial administration: Supreme Court's original jurisdiction judicial administration: review of non-final order judicial administration: change in state law (cf. no merits: remand to determine basis of state court decision) judicial administration: federal question (cf. no merits: dismissed for want of a substantial or properly presented federal question) judicial administration: ancillary or pendent jurisdiction judicial administration: extraordinary relief (e.g., mandamus, injunction) judicial administration: certification (cf. objection to reason for denial of certiorari or appeal) judicial administration: resolution of circuit conflict, or conflict between or among other courts judicial administration: objection to reason for denial of certiorari or appeal judicial administration: collateral estoppel or res judicata judicial administration: interpleader judicial administration: untimely filing judicial administration: Act of State doctrine judicial administration: miscellaneous Supreme Court's certiorari, writ of error, or appeals jurisdiction miscellaneous judicial power, especially diversity jurisdiction