The Federal Communications Commission, clarifying its landmark decision last April to rewrite the rules on indecency on radio and television, yesterday gave stations effective license to broadcast indecent materials between midnight and 6 a.m.

The commission rejected requests that it detail what it would consider to be indecent, saying that broadcasters must decide for themselves based on a 25-word definition the commission endorsed last spring.

The midnight-to-6 a.m. "window" was adopted, the commission said, because at that time children are unlikely to be in the audience. "After midnight, the onus will be on parents to supervise their children," said FCC general counsel Diane S. Killory.

The National Association of Broadcasters (NAB) criticized the ruling, suggesting it was "too vague." Civil liberties activists, meanwhile, said it restrained free speech, and an antipornography group said it "pushed the pig into the parlor of millions of American homes."

FCC Chairman Dennis Patrick rejected suggestions of damage to free speech. "I'd put this commission second to none in terms of our fidelity to the First Amendment," he said before the commission voted 4 to 0 to enact the new plan.

Last April the FCC abandoned an 11-year-old practice of defining indecency on the basis of "seven dirty words" made famous in a case, eventually heard by the Supreme Court, against the radio broadcast of a routine by comedian George Carlin in which the words were used.

It instead said that indecency would henceforth be identified using a 1978 Supreme Court definition as "material that depicts or describes, in terms patently offensive as measured by contemporary community standards for the broadcast medium, sexual or excretory activities or organs." The words used would not matter.

The April decision grew out of FCC proceedings against three radio stations over sexually explicit material being broadcast by them. The three were KPFK of Los Angeles, KCSB of Santa Barbara, Calif., and WYSP of Philadelphia. WYSP was cited for early morning broadcasts by former Washington radio talk show host Howard Stern. During those proceedings, the FCC developed the new standard and ruled that the stations would have violated it. But the FCC only issued warnings because the new standard had not been in force at the time of the broadcasts.

In legal terms, indecency is different from obscenity, for which the FCC applies a three-part definition adopted in 1973 by the Supreme Court. Obscenity is banned from the airwaves. Indecency can be used, but only at times when there is not a reasonable chance that children will be exposed to it.

After the April decision, broadcasters complained that the FCC had not only given them a vastly broad definition of indecency, it had not given guidance on how to decide whether children might be in the audience. In subsequent filings, broadcasters asked that a general rule be established allowing airing of the material after 10 p.m.

The FCC, however, yesterday rejected that as too early, and set midnight as the starting point.

The FCC refused to give a list of words or pictures that would be considered indecent at all times, saying that "context" was key to any determination. It did say that any "merit" -- whether artistic, political or other -- found in a program would be considered in deciding whether the program was indecent. But it ruled out promising that any program found to contain merit of some kind would automatically be ruled not indecent.

General counsel Killory said broadcasters should decide for themselves whether material was indecent under the April standard.

The NAB welcomed the time specifications. "At least the FCC saw fit to establish a safe harbor at midnight," senior vice president Jeff Baumann said through a spokesman. "But based on what was said at today's meeting, we have grave concerns as to whether or not the FCC's indecency policy is too vague and we will have to examine the document very carefully to determine whether legal action is in order."

Morality in Media, a New York-based group fighting pornography, said the FCC had "pushed the pig into the parlor" by creating the post-midnight window and had overstepped its authority. "The federal broadcast law, passed by Congress -- not by the FCC -- prohibits obscene and indecent broadcasting and has no window," it said. "It applies 24 hours a day."

Civil liberties groups last April faulted the new way of defining indecency. Barry W. Lynn, legislative counsel for the American Civil Liberties Union, called yesterday's action "a totally inadequate resolution for this matter."

"The heart of the problem with the April decision was that they present a very fluid definition of indecency and thus tend to pursuade broadcasters not to deal openly with sexual topics, either humorously or seriously.

"Any sexual allusions could be subject to FCC administration action under that rubric, and that is very dangerous in a free society," Lynn said.