The decision, by a divided panel of the United States Court of Appeals for the Second Circuit in New York, was a sharp rebuke for the F.C.C. and for the Bush administration. For the four television networks that filed the lawsuit — Fox, CBS, NBC and ABC — it was a major victory in a legal and cultural battle that they are waging with the commission and its supporters.

Under President Bush, the F.C.C. has expanded its indecency rules, taking a much harder line on obscenities uttered on broadcast television and radio. While the judges sent the case back to the commission to rewrite its indecency policy, it said that it was “doubtful” that the agency would be able to “adequately respond to the constitutional and statutory challenges raised by the networks.”

The networks hailed the decision.

“We are very pleased with the court’s decision and continue to believe that the government regulation of content serves no purpose other than to chill artistic expression in violation of the First Amendment,” said Scott Grogin, a senior vice president at Fox. “Viewers should be allowed to determine for themselves and their families, through the many parental control technologies available, what is appropriate viewing for their home.”

Mr. Martin, the chairman of the commission, attacked the panel’s reasoning.

“I completely disagree with the court’s ruling and am disappointed for American families,” he said. “The court says the commission is ‘divorced from reality.’ It is the New York court, not the commission, that is divorced from reality.”

He said that if the agency was unable to prohibit some vulgarities during prime time, “Hollywood will be able to say anything they want, whenever they want.”