William H. Rehnquist is generally regarded as among the shrewdest of the nine Supreme Court justices. He is also known inside and outside the court as one of the least receptive to women's rights causes.

So having him write a major opinion in a sex, discrimination case is like putting Chrysler in charge of Japanese import controls or Rep. Henry J. Hyde in charge of an abortion clinic.

Having him write that opinion when a second case, a related and even more important one, is pending takes the equation one step further. The first opinion inevitably influences the second or even, as in politics, serves as a stalking horse.

Rehnquist was assigned to write the opinion in the first case, a sex discrimination challenge to California's statutory rape law, Michael M. v. Superior Court of Sonoma County. The law punishes men for sexual intercourse with underage women but does not punish women for sex with an underage male. The opinion was issued March 23.

The second case, Rostker v. Goldberg, is the constitutional attack on the all-male draft registration program, still under consideration by the court. The lawyers supporting that attack are, as people say after tornadoes, still sifting through the rubble assessing the damage.

They hope the fact that only three other justices (Chief Justice Warren E. Burger, Potter Stewart and Lewis Powell) joined Rehnquist's opinion limits the impact. The fourth, Harry A. Blackmun, agreed with the judgment but wrote his own concurrence. He did not, however, question Rehnquist's legal reasoning.

Before the mid-'70s, governments at any level could and did differentiate in the law between men and women pretty much as they pleased. If challenged, officials needed only provide the thinnest of justifications. Women, for example, could be barred from working as bartenders before they were considered too tender for such a boisterous environment.

Then the Supreme Court began requiring a different kind of justification, not based on stereotypes but on fact and need. Was the basis for a distinction real, the courts started asking, and was the distinction really necessary to achieve some important purpose? The new standard resulted in the elimination of hundreds of laws treating men differently from women.

It was the basis for the attack on California's statutory rape law. And it was the basis on which a three-judge panel in Philadelphia ruled the draft registration program unconstitutional. "The complete exclusion of women from the pool of registrants does not serve 'important governmental objectives' and is not 'substantially related' to any alleged government interest," the panel said last July.

And it was also the target of Rehnquist's March 23 opinion in Michael m. v. Superior Court of Sonoma County.

California is one of 13 states with statutory rape laws that differentiate between men and women. A man can be prosecuted and imprisoned for sexual intercourse with a woman under 18. But no law subjects a woman to punishment for sexual intercourse with an underage male. Rehnquist upheld the law's constitutionality. He and his colleagues accepted the state's justification: that the gender distinction was designed to prevent teenage pregnancies; that women, not men, can become pregnant and women, not men, need the protection of the law.

The potential impact on future sex-discrimination cases comes not so much from Rehnquist's discussion of pregnancy as it does from his discussion about how to approach gender distinctions in general.

Until March 23, most lawyers thought the standard for judging such cases was by now well-established by the court. Not so, said Reinquist. "This court," he wrote, "has had some difficulty in agreeing upon the proper approach and analysis in cases involving" gender distinctions.

Yes, he said, these distinctions deserve a closer look than others, perhaps a "somewhat sharper focus."

To laymen, it may be all a matter of words. To lawyers, otherwise subtle semantic differences can swing a case or thoroughly confuse it. It's a matter of degree: a justification must be exceedingly persuasive under previous rulings and maybe not so persuasive under Rehnquist's.

There were other parts of Rehnquist's opinion that seemed tailored to the draft case.

Both the statutory rape challenge and the registration challenge, though championed by women, were brought by men as "victims" of discrimination. "We find nothing," Rehnquist wrote in the rape case, "to suggest that men, because of past discrimination or peculiar disadvantages, are in need of the special solicitude of the courts."

Both the rape case and the draft case featured debates about the legitimacy of justifications for gender distinctions provided by lawyers years after the laws were enacted. In the draft case, the original motives of Congress were clearly laced with the sort of sexual stereotypes (men won't serve under women in the Army) that are now unacceptable to the courts. The challengers of the law want those old arguments to be the ones considered by the justices because they can easily be challenged.

In 1980, when Congress rejected President Carter's proposal to include women in the draft registration, it and government lawyers came up with a new, more acceptable, justification: "military flexibility." The government wants the court to use the new arguments.

"The only question under the Federal Constitution," Reinquist wrote on March 23, "is whether the legislation violates the Equal Protection Clause of the Fourteenth Amendment, not whether its supporters may have endorsed it for reasons no longer generally accepted."

"Some people are saying it was written with the draft case in mind," said Isabelle Katz Pinzler of the American Civil Liberties Union, a lawyer on the team challenging the all-male registration. "I don't know whether it's true or not.

"But it certainly doesn't bode well. I think that in Michael m., we may have turned the corner and be headed backwards."