In a spirited argument before the Supreme Court yesterday, a lawyer for Playboy Entertainment described a federal law designed to block sexually explicit cable programs from child viewers as "regulatory overkill." But a government attorney countered that without the law in place, graphic adult fare could be glimpsed by anyone "merely with the flip of a dial."

The case involves cable channels whose signals "bleed" into the homes of nonsubscribers, and it arises against the backdrop of increasing societal concern about the sex and violence children see on television. Such issues were clearly on the minds of some justices, who referred to busy parents unable to monitor what their children watch. Justice Stephen G. Breyer observed at one point that unlike earlier generations, "many, many thousands of children come home from school and there's no one there."

As part of the wide-ranging 1996 telecommunications law, Congress required cable operators offering sexually explicit content to "fully scramble" their signals or to show such programming only between 10 p.m. and 6 a.m. But last year a special district court in Delaware said the provision violated the First Amendment's free speech guarantee. The court noted that the law requires complete scrambling even to households without children and, because of the technical difficulty of such scrambling, effectively imposes a ban on adult cable programming for most of the day. The court suggested the government could have opted for the less restrictive alternative of requiring cable operators to notify parents that they may obtain "lockboxes" that fully block audio and video signals.

In the government's appeal yesterday, Assistant Solicitor General James Feldman emphasized children's boundless access to indecent cable programming, which he said can be seen and heard because of inadequate scrambling in the nonsubscriber homes of some 29 million children.

He acknowledged that cable generally would be given a high level of free-speech protection but said the court should defer to Congress's judgment in this area because it concerns children. Feldman added that given the proliferation of videocassette recorders in homes and the ease with which adults could arrange to tape sexually explicit fare showing after 10 p.m., "people who want to watch it at other times can watch it."

Robert Corn-Revere, representing Playboy, emphasized that the court has accorded cable television great First Amendment protection, and to compromise here, he said, "would be a significant change in the law." He said the 1996 provision was too broadly written, sweeping in everything from hard-core pornography to information about safe sex practices. He acknowledged the government's interest in preventing children's access to sexually oriented programs, but said it does not justify widely suppressing speech aimed at adults.

Corn-Revere also contended that giving parents an option to block out signal bleed was a legitimate alternative and that it would not be hard for operators to notify parents of their ability to obtain the free service.

In their questions yesterday, the justices spent much of the time trying to straighten out some key factual issues in the dispute, chiefly what sort of "sexually explicit" programming is restricted under the law and whether it is too broadly defined.

Justice Antonin Scalia suggested that most of the tapes the Justice Department put in evidence were so obscene that the government would rightly ban them, but he questioned whether some less offensive shows would be curtailed by the law.

The justices also expressed doubts about whether "lockboxes" are a viable alternative. They questioned how easily parents could obtain the boxes, whether they would involve a service call and if the option would ultimately prove so expensive for cable operators that they would later claim that this requirement was unconstitutionally restrictive, too.

"Many parents are not going to know about this," remarked Justice Anthony M. Kennedy. "They're not going to do anything about it."

A ruling in the case of United States v. Playboy Entertainment Group is expected by next summer.