John Belushi and Dan Aykroyd, two of the stars of NBC's popular late-night comedy series "Saturday Night Live," have quit the show. They will not be in the cast when the program begins its fifth season on Oct. 13.

Belushi and Aykroyd, in Chicago filming "The Blues Brothers" for Universal, made a joint announcement late yesterday of their decisions to leave the program that has made them household names, comic heroes to much of America's youth, and fledging movie stars.

Both cited the pressing demands of other (and more lucrative) movie and record offers as among their reasons for leaving.

Reached in Los Angeles, producer Lorne Michaels, who created the program and picked its original cast, said of the departure of Aykroyd and Belushi, "It's a chilling thought not to have them there, but I want the best to happen to them as well as the best to happen to the show. I only have nice things to say about them. They worked very hard."

Michaels said the new season will begin with only the five remaining cast members and that additional players will be added gradually so they will not look like "the new John Belushi and the new Danny Aykroyd." That was the policy when the first superstar to emerge from the cast, Chevy Chase, left the program during its second year.

"I'm sure it's career conflict stuff rather than a desire not to do the show," Michaels said of the announcement by Belushi and Aykroyd. "They'll always have a home on the show. In fact we're turning both of their offices into shrines. I'll be very hurt if they're ever in New York and don't call me."

"Sometimes success can be very bittersweet," Belushi said in a statement. He recalled waiting many years for his big break on TV -- appearing in such satirical stage shows as "National Lampoon's Lemmings" -- and said, "Now I'll move on and make way for some other new talent to step in and make his own mark."

Both statements seemed to leave a door open for future guest appearances on the show. Belushi, after praising the show and the network, said, "I undoubtedly will be doing TV again, and like a homing pigeon, I'll be back at 30 Rock," a reference to NBC's New York address, 30 Rockefeller Plaza.

Aykroyd, who has also been on the show's ample and mostly young writing staff during his four years in the cast, said he would maintain a "24-hour home hot-line" to the "Saturday Night" office and, although "there comes a time for old warhorses to haul water," he added that "if the boys write a great President Carter scene, I guess they could talk me into coming on and playing him."

President Carter was among a number of impersonations perfected by Aykroyd during his years on the show. He also did an imitation of Tom Snyder that probably did much to increase Snyder's own popularity. With Jane Curtin, Aykroyd frequently parodied the "Point-Counterpoint" segment of the CBS news show "60 Minutes," with Aykroyd always beginning his rebuttal, "Jane, you ignorant slut."

Belushi's contributions ranged from a very fat bee in the early days of the program to a deranged samurai swordsman discovered in such unlikely livelihoods as running a delicatessen or a dry cleaners or holding forth at the disco in a sketch called "Samural Night Fever." Belushi proved himself one of the most agile roly-poly actors in television history with such specialties as a hysterical impression of rock star Joe Cocker.

Belushi and Aykroyd became close friends and discovered a mutual interest in blues music. This led to the creation of a mythical, soulful singing dus, "The Blues Brothers," and a record album that, though cut as a lark, has sold more than 2 million copies and also evolved into the movie now shotting in Chicago and co-authored by Aykroyd.

They also established a private, after-hours, Greenwich Village bar where each week "Saturday Night Live" turned into Sunday Morning Drunk for Belushi, Aykroyd, members of the cast, and friends.

Belushi's career took a great leap upwards in 1978 with the enormous box-office success of the Universal comedy "National Lampoon's Animal House," a farce about a slobbola fraternity that inspired a number of movie and TV imitations and boosted Belushi's asking price per picture from $35,000 to $350,000 minimum.

Aykroyd will soon be seen with Belushi in their first on-screen pairing, Steven Spielberg's World War II comedy "1941." Aykroyd said of the TV show he is leaving that it is "the greatest creative arena in the medium" and noted that the kind of freedowm enjoyed by the cast and writers "is not the situation in the videotape mills of Hollywood."

Belushi, Aykroyd and others in the show's repertory company were once known as the "Not Ready for Prime Time Players," but the appellation was dropped last year. Now Belushi and Aykroyd are not only ready for, but too expensive for, prime time.