A requirement that American diplomats retire at age 60 was upheld yesterday by the Supreme Court.

The justices said that the requirement -- in the Foreign Service Act of 1946 -- doesn't deny the equal protection of laws guaranteed by the Constitution even though there is no mandatory retirement age for Civil Service employes, including those who serve abroad with diplomats.

The 8-to-1 ruling affects about 8,000 members of the Foreign Service and others covered by its retirement system.

A group of former and present participants in the system had tried to overturn the requirement of compulsory retirement at 60 with a lawsuit asserting that it has no relation to the objective of reliable service in important posts abroad.

Overseas conditions often are not more strenuous than those in the United States, they argued in affidavits. And, they said, one's arrival at age 60 is in itself insufficient to show a decreased physical and mental potential.

But the government argued that the 60-and-out rule serves one of its legitimate and rational goals: assuring the competence and the mental and physical reliability of those who hold positions critical to U.S. relations with other countries, who sometimes serve under difficult and demanding conditions and who must be ready at any time to take assignments under such conditions.

Despite their differences, both sides as well as a panel of three U.S. District Court judges here agreed that whether the retirement requirement denies equal protection must be determined under a 1976 ruling involving a Massachusetts state trooper.

In that decision, the court said that to be valid, mandatory retirement at a certain age -- 50, in the trooper's case -- must be "rationally related to a legitimate state interest."

The panel ruled for the plaintiffs. In doing so, it rejected additional State Department arguments such as a claim that compulsory 60-and-out serves the law's primary goal by creating predictable opportunites for promotions that spur morale and stimulate superior performance in the ranks.

In the opinion for the court overruling the panel, Justice Bryon R. White wrote that the judgment that the Foreign Service needs mandatory retirement at 60 more than other parts of the government is one for Congress to make.

"Since the congressional judgment to place a high value on the proper conduct of our foreign affairs can hardly be said to be constitutionally impermissible, it was not for the District Court to refuse to accept it," least of all to say, as it did, that it is "blatantly arbitrary and irrational," White said.

He noted that among Civil Service employes, only 5 percent serve overseas at any time and that for them "foreign duty is in the main a voluntary matter."

Justice Thurgood Marshall, who dissented in the 1976 case, dissented again, saying the retirement rule "cannot withstand close scrutiny."