The two hosts would be forced to perform without writers and might have to face opposition from the Writers Guild, which has thus far urged the late-night hosts to support the strike by staying off the air. However, the executive producers of both shows said yesterday that they believed the tone of that opposition has softened in recent weeks after the hosts began paying the salaries of the nonwriting staff members of their shows.

"I think the temperature may have changed a little," said Jeff Ross, the executive producer of Mr. O'Brien's show.

CBS's late-night star, David Letterman, is pursuing an interim agreement with writers that would allow him to return on Jan. 2 with his writers. Mr. Letterman is positioned to make such a deal because his production company, Worldwide Pants, owns both his show and the one that follows on CBS, which has as its host Craig Ferguson.

This weekend Mr. Letterman's representatives said they had been negotiating with the guild for that interim agreement and were hopeful they would be successful, but the deal was not yet settled. One representative of a late-night show said that some members of the guild leadership may have concerns about making a separate arrangement with Mr. Letterman, and an agreement with the guild is far from a sure thing.

The plans of several other late-night hosts remained unclear. A spokesman for the Comedy Central channel said that no decision has been made yet about when or whether its two hosts, Jon Stewart and Stephen Colbert, will return without writers. An ABC spokeswoman said a decision about when its late-night star, Jimmy Kimmel, might return remained day to day.

In statements released yesterday, Mr. Leno and Mr. O'Brien both said they have supported the strike, but decided the time had come to return because of the risk that their nonwriting staff would lose their jobs.

Mr. Leno said, "I remained positive during the talks and while they were still at the table discussing a solution 'The Tonight Show' remained dark in support of our writing staff. Now that the talks have broken down and there are no further negotiations scheduled, I feel it's my responsibility to get my 100 nonwriting staff, which were laid off, back to work."

Mr. O'Brien said, "With the New Year upon us, I am left with a difficult decision. Either go back to work and keep my staff employed or stay dark and allow 80 people, many of whom have worked for me for 14 years, to lose their jobs. If my show were entirely scripted I would have no choice. But the truth is that shows like mine are hybrids, with both written and nonwritten content. An unwritten version of 'Late Night,' though not desirable, is possible and no one has to be fired." He added, "I will make clear, on the program, my support for the writers and I'll do the best version of 'Late Night' I can under the circumstances."

Exactly what the shows will be able to do is unclear at this point, the executive producers said. Debbie Vickers, the executive producer of the "Tonight" show, said that she hoped Mr. Leno would still be able to perform some kind of comedy monologue, his show's trademark. "I would like to not rule that out," Ms. Vickers said.

But the shows are not certain what kinds of rules the Writers Guild might try to impose on the hosts, both of whom are members of that union. Mr. Ross said, "It going to be hard. We have 12 writers and they come up with comedy material every day." But he said he was confident in Mr. O'Brien's ability to fashion an entertaining show anyway.

Ms. Vickers noted that both men have had a lot of experience improvising comedy from conversation on the shows. "They're comedians," she said. She added: "When Jay is challenged, he rises to the occasion. Hopefully when we get there we'll have some fun and some moments that are unpredictable."

One possibility for both shows is the addition of more guests. That area is also open to some question, however, because it is still uncertain whether some Hollywood stars will decline to appear on the shows with the strike still on. Mr. Ross said that "early on" during the strike, potential guests had indicated real reluctance to cross picket lines to appear. Lately, he said, that has been less of a factor when the shows have contacted publicists for stars.

But NBC's shows will clearly be at a disadvantage if they return without writers while Mr. Letterman's shows are fully written. "It's not going to be an even playing field," Mr. Ross said.

The Jan. 2 date would put the amount of time the NBC hosts stayed off the air in support of the strike at about two months. In the previous writers' strike in 1988, Johnny Carson, the legendary host of the "Tonight" show, came back onto the air without writers after staying out two months.

Rick Ludwin, the NBC executive in charge of late night programs, made the connection directly during yesterday's news conference, noting that the guild permitted Mr. Carson to write material for himself when he came back on the air. He said he hoped Mr. Leno and Mr. O'Brien would receive that same courtesy.

The writers reacted with anger when two talk hosts, Ellen DeGeneres and Carson Daly, returned to their shows. Mr. Daly even had hecklers disrupting his tapings. But the NBC hosts, along with Mr. Letterman, Mr. Kimmel and the Comedy Central hosts, have all won praise from the writers for staying off the air so long and for paying for their staffs out of their own pockets.