The Supreme Court agreed Monday to rule for the first time in 30 years on what constitutes indecency on broadcast television and radio.

The justices will weigh whether federal regulators may levy large fines on broadcasters who let expletives on the airwaves during daytime and early evening hours.

The court could rule that the Federal Communications Commission has broad power to decide what is acceptable for broadcasts. Or the justices could conclude that the 1st Amendment's protection for the freedom of speech does not allow the government to punish broadcasters for an occasional vulgarity.

The justices have not ruled on the indecency standard since 1978, when they upheld fines against a radio station for broadcasting comedian George Carlin's "seven dirty words" monologue in midafternoon. One justice described Carlin's performance as "a sort of verbal shock treatment" because the familiar curse words were repeated over and over.

Since then, however, it has been unclear whether the use of a single expletive could be judged indecent. Federal law forbids broadcasting "any obscene, indecent or profane language," but Congress has left it to the FCC and the courts to define indecency.

Last year, the major networks won a ruling in New York that blocked the FCC from enforcing a strict new rule against the broadcasting of "fleeting expletives."

Bush administration lawyers urged the high court to take up the dispute and to give the FCC a green light to enforce its crackdown on vulgar words. The government says broadcasters who use the public airwaves have a duty to protect children and families from unexpectedly hearing foul language.

The FCC has fined CBS $550,000 for broadcasting Janet Jackson's performance at the 2004 Super Bowl, which included a brief exposure of her breast. The network is appealing the fine in a court in Philadelphia.

The president of the Parents Television Council in Los Angeles applauded the court's announcement. "Such harsh, unedited profanity is unacceptable for broadcast over publicly owned airwaves when children are likely to be watching," said Timothy Winter, the president.

His group claims more than 1.2 million members, and he said many complained when they heard expletives used during Hollywood award shows. "It seems you can't have an awards show without someone dropping an F-bomb," Winter said.

The FCC cited several incidents that led it to issue the new rule. Singer Bono of U2 exulted upon winning a Golden Globe for an original song, calling it "really, really f . . . brilliant." Entertainer Cher described a career achievement award on another program as a rebuke to her critics. "So, f . . . 'em. I still have a job and they don't," she said.

The major TV networks sued to block the rule. In their defense, they say they have firm policies against the use of vulgar words. They are not included in scripts, for example. But on occasion, they say, these words have slipped passed monitors and gone on the air when a guest performer appeared on a live broadcast.

The networks used a five-second delay on several of the live broadcasts cited by the FCC, but a monitor failed to bleep out the expletive.

The stakes for broadcasters increased when Congress voted in 2006 to raise the maximum fines for indecency tenfold. Network executives say they could face millions of dollars in fines for letting a single expletive go on the air during a national broadcast.

In March 2004, shortly after the Janet Jackson incident, the FCC adopted its zero-tolerance policy for "fleeting expletives." The commissioners rejected the defense that Bono had used the F-word as an adjective, not a curse.

The U.S. appeals court in New York, in its ruling last year, agreed with the broadcasters that the FCC had not justified its abrupt change in policy. Its judges also said the policy was unclear because the F-word was permitted in some news shows and in the TV broadcast of "Saving Private Ryan." The commissioners said the profanity on the D-day beaches was integral to depicting the horror of war.

Lawyers for the broadcasters had urged the Supreme Court to steer clear of the case. They said the FCC should be forced to reconsider and clarify its policy.

"I thought there was no chance they would take this case," said Andrew Jay Schwartzman, president of the Media Access Project. "If the FCC is affirmed, the message will be that indecency is whatever the FCC says it is."

It is not clear whether the growth of new media will affect the court's view of what is indecent. Since the court last ruled on the issue, cable TV, the Internet and satellite radio have emerged as competitors to traditional broadcasters. But these new media are not regulated by the FCC because they do not transmit signals over the public airwaves. Arguments in the FCC vs. Fox TV will be heard in the fall.

The court also agreed to decide whether the special protections added to the Voting Rights Act in 1982 extended to communities where blacks make up less than half the population.

The outcome in Bartlett vs. Strickland could affect how electoral districts are drawn after the 2010 census. The law forbids states from splitting up large blocs of black voters who could elect a black representative to a state legislature or Congress. But it is not clear whether this protection is triggered only if blacks make up a majority of the community.

The court will also decide in Melendez-Diaz vs. Massachusetts whether criminal defendants have a right to question lab technicians whose reports are used by prosecutors. In recent years, the court has stressed that defendants have a right to confront all witnesses.