NBC's "Saturday Night Live," the popular and irreverent late-night comedy series, will be back for a sixth season next year, but perhaps in name only. Lorne Michaels, the original producer and creator of the show, has quit the job and a new producer, Jean Doumanian, took over earlier this week.

Insiders expect that with the exit of Michaels, many of the show's resident cast members will leave as well. All of their contracts with NBC were up as of this season's last show, which aired May 24 and which concluded with guest host Buck Henry saying, "Good night, and goodbye." This was followed by a flashing "On Air" sign that went dead.

An NBC statement yesterday said negotiations with "several" of the cast members are under way, but privately sources indicated it is unlikely that Gilda Radner, Bill Murray, Laraine Newman or Jane Curtin will return to the show next year.

A new, "exclusive, long-term production agreement" with Michaels is being negotiated by NBC, the statement said. Michaels said final details of that agreement have not been reached. "It is my intention to stay with NBC," he said. Michaels. who invented the program, hired its original cast, and insisted it be done -- against the prevailing trend -- live from New York and not on tape from L.A., said that leaving the program is a "painful experience" and that he planned to embark immediately on "a fallow period."

He will spend the summer "writing and taking long walks," Michael said.

NBC said a new producer would be: announced shortly, but Doumanian confirmed that she has already been hired. Associate producer of the program since its 11th show, she was in charge of choosing musical acts to apper during breaks in the comedy.

Doumanian said she could not predict which cast members would return but did say she will be "looking for new talent" and "gathering a new crew." Several of the writers hired by Michaels are expected to bail out during the current upheaval as well.

But Doumanian said she is looking forward to putting the new show together. "I find it so exciting, I can't tell you," she said. "I want to really retain the stuff that made the show great, and try to introduce the element of growth. It needs new elements, and once they're added and new things will evolve, it will be wonderful."

She praised Michaels for "the terrific legacy" he has left her and said she hopes to keep the program "as outrageous as ever."

Brandon Tartikoff, president of NBC Entertainment, also praised Michaels, in a prepared statement, for having left "an indelible mark on late-night" TV and for being "the first television pioneer of the television generation."

Unveiled with barely a peep of fanfare in 1975, "Saturday Night Live" went on to become one of the most influential TV hits of the decade and catapulted such cast members as Chevy Chase, John Belushi, Dan Aykroyd, Radner and Murray into national stardom. It also made a household doodad out of a much-maligned lump of clay known as "Mr. Bill."

The program finished its first season with a cumulative rating of 6.6 and a 23 percent share of the viewing audience. It finished its fifth season with a 13.5 and 38 share -- the highest in late-night television. ABC has already unveiled its own "Saturday Night" imitation, "Fridays," and CBS has several late-night comedy projects in development.

According to an internal NBC memo obtained by Variety magazine, the program cleared $8 million in profits this season alone, not including those made from a series of prime-time reruns. Insiders estimate NBC's total take on the first five years at around $50 million. Considering the fact that RCA, NBC's parent corporation, will soon market its Selectavision video disc system and, that it owns the rights to all "Saturday Night" broadcasts, those profits could be just beginning.

But for Michaels, it's the end. "The hardest thing in comedy," he said yesterday, "is knowing when to leave the stage."