The Supreme Court, ending its 1981-82 term, yesterday relaxed its ordinary constitutional protections for free speech to allow states to move against child pornography, which the justices called a "serious national problem."

In a 9-to-0 judgment, the court upheld a New York law that makes the production and distribution of any pictures of children under 16 engaged in sex or lewd conduct criminal acts, whether or not the material is considered legally obscene.

The absence of an obscenity requirement, the court's usual way of separating art from pornography, was the controversial part of the law, since critics contended that it opens the door to censorship of pictures in art galleries, medical books and literature, as well as pornography.

The court tacitly acknowledged that yesterday, but it said that in this one area, the states "are entitled to greater leeway."

"The prevention of sexual exploitation and abuse of children constitutes a government objective of surpassing importance," Justice Byron R. White wrote for the court. "We shall not second-guess" New York's legislative judgment.

The ruling supports the laws of 13 states and will undoubtedly encourage others that have not gone as far, including Maryland and Virginia, to go further.

In other action on the court's last day until October, the justices limited imposition of the death penalty to criminals directly responsible for murder Details, Page A14 , narrowed the power of the states over their own ground water Details, Page A10 and told a federal judge in Arkansas that he had no business intervening to block expulsion of a high school student who was drunk in school Details, Page A11 . The court postponed until next term a ruling on the constitutionality of the legislative veto.

The New York "kiddie porn" case was being watched carefully by the publishing industry and First Amendment activists as well as by organizations campaigning for controls on obscenity.

It confronted the justices with the difficult choice of sticking to their conventional rules and striking down the statute, risking a nasty public backlash, or of letting the law stand with the resulting erosion of previous protections.

The first clue to their intentions came in April during oral arguments in which justices openly bristled with outrage at the lawyer for the defendant in yesterday's case.

New York authorities had charged Paul Ira Ferber with violating the "kiddie-porn" law after he sold two films, "devoted almost exclusively to depicting young boys masturbating," the opinion noted, to an undercover police officer.

The New York Court of Appeals reversed his conviction, saying the law reached too far and prohibited the distribution of materials like medical books and educational guides dealing "with adolescent sex in a realistic, but nonobscene manner."

The New York law bans any promoting, distributing, or producing of pictures, films, dances or plays showing children under 16 engaged in sexual acts or lewd exhibitions. But under the court's general guidelines for censorship, materials may be banned only after being judged "legally obscene," appealing to the "prurient interest of the average person" and "patently offensive" according to local community standards.

Yesterday, White said those guidelines were inadequate for controlling child pornography. He acknowledged that the laws "run the risk of suppressing protected expression by allowing the hand of the censor to become unduly heavy."

He also said it might be used to censor materials not legally obscene. But he said these materials do more than just offend. Their very production exploits and injures the children involved and leads to their abuse.

"The prevention of sexual exploitation and abuse of children constitutes a government objective of surpassing importance," he said. ". . . . The use of children as subjects of pornographic materials is harmful to the physiological, emotional and mental health of the child. That judgment, we think, easily passes muster under the First Amendment."

Chief Justice Warren E. Burger and Justices Lewis F. Powell Jr., William H. Rehnquist and Sandra D. O'Connor joined White's opinion. O'Connor agreed with the opinion but wrote separately to say that the ruling does not exempt material "with serious literary, scientific or educational value."

Justices William J. Brennan Jr. and Thurgood Marshall wrote separately to say that the ruling does exempt such materials. Justice John Paul Stevens, in his own statement, also said the decision was limited. And Justice Harry A. Blackmun said he concurred in the result but expressed no view on the opinion.

Michael A. Bamberger, who wrote a friend-of-the-court brief for the American Civil Liberties Union, the American Booksellers Association and other concerned groups who fear "overbroad" laws, was critical of the ruling. "It chills the rights of those who publish and sell materials protected by the First Amendment," he said.

"The fact that they leave the statute sitting there creates a danger all by itself, because it covers more than child pornography," Bamberger said. "There are people about the land who feel strongly about the whole concept of sex education, for example, and could use this as a tool to limit the distribution of serious materials."

Bamberger also expressed concern that the court's willingness to carve out another exception to First Amendment protections presaged a "freer view of the ability to create" further exceptions.

"If you start talking about the compelling interest in child pornography, a court may follow and say a state has just as compelling an interest in national security, and carve out a new exception that hadn't been there before," Bamberger said.