The Supreme Court declined yesterday to let the Reagan administration participate in oral arguments this fall in two major cases involving abortion and affirmative action.

The decision means that the court will not hear Justice Department lawyers argue that the court's 1973 decision legalizing abortion should be overruled.

The department, in a controversial brief filed with the court, took that position, but neither plaintiff in the cases from Illinois and Pennsylvania have asked the court to reverse that decision.

Department officials said yesterday the court's action, which denies the administration a highly visible platform to press its views, should not be seen as a rebuff of the administration's position. A senior official emphasized that the court has rarely granted requests for additional time to argue cases.

The Illinois and Pennsylvania cases, which involve state efforts to limit abortions, are to be argued on Nov. 5.

The affirmative action case, which involves a teachers' collective bargaining agreement in Jackson, Mich., will be argued the next day. The court is expected to rule on a union contract that permits layoffs of white teachers to retain black teachers with less seniority.

When the federal government is not directly involved in a case, it must secure the consent of those involved, and then obtain the court's permission, to argue a case.

If those involved agree, the court generally, as a matter of respect or courtesy, allows the Justice Department to argue its views.

Department officials said that in these cases, neither the Pennsylvania nor Michigan officials agreed to relinquish part of their allotted half-hour of argument.

The Supreme Court justices, who might have granted additional argument time given the importance of the issues, did not reveal why they denied the requests.