Every year Lorne Michaels, the producer of NBC's "Saturday Night Live," calls up Johnny Carson and asks him if he would like to be a guest host on the show. Johnny always says he's sorry, but no. If he gets the offer again, however, old John ought to grab it, because he could pick up a few million more viewers just like that.

Ratings for "The Tonight Show Starring Johnny Carson" have eroded steadily since 1974. Ratings for "Saturday Night Live," an NBC executive recently announced, are up 37 percent over last year. Commercial time on the program is sold out for 1979, so sponsors are now lining up to buy time in the first quarter of 1980.

Johnny's highest rated show of the past season was his annual anniversary number, which got a 12.6 Nielsen and attracted 22 million viewers, according to NBC research. The biggest "Saturday Night" show, with Kate Jackson as guest host on Feb 2, and a 15.1 rating and was seen by an estimated 28 million people. For NBC, Saturday has become a night in shining armor while carson, once the prince of darkness, now gets beaten by returns of old cop shows on other networks.

Naturally NBC President Fred Silverman showed up at the big party thrown at the foot of the Rockefeller Center fountain when "Saturday Night" called it a season, a fourth season actually, last Saturday. Conspicuously, this was not one of the programs on which the cast of cut-ups took any jabs at Silverstreak. "He told me, 'I was surprised you didn't throw any shots at me,'" says Michaels, "and I told him, 'I would never do it in front of you.'"

On one show, John Belushi played Silverman as a kind of vacillating monarch of the mediocre, at one point accused by associates, because of the dreadful spring schedule he'd unveiled, of secretly being a "double agent" for his old boss, ABC. Michaels confesses that joke was born in a wisecrack made by Paul L. Klein, a former NBC executive vice president with whom Silverman had enjoyed innumerable celebrated shouting matches. Klein left the network in mid-season and everyone stressed the amicability of it all.

From his office in New York, Fred Silverman chuckles at the notion that he has ever had a fit over something said about him on "Saturday Night Live." He has more important things to lavish fits on. "They talk about people who are, for whatever the reason current," says Silverman. "Whether it's Jimmy Carter or whoever it is. I don't know whether most people know who Fred Silverman is, but for the kids on 'Saturday Night,' the whole world is a target."

As for the absence of Silverman barbs on the last show of the season, "I really was very surprised," Silverman says. "I thought I would walk out of the studio bloody." Why, he enjoys the fame the ribbing has brought him, he contends, "All my wife's family live on Long Island, and they finally think I'm a celebrity because of 'Saturday Night Live.' Before it was always 'Fred who?'"

Both Silverman and Michaels deny published rumors that NBC brass have been begging the cast members of "Saturday Night" to do prime-time programs for the network and been turned down. "For a while it looked like Gilda Radner would do a show," says Silverman, "but then she didn't want to. And we want to keep 'Saturday Night' a healthy show, and not stage any raids on it."

Michaels looks back on the fourth season as a good one but says he wants to make major changes for the fifth. One change he doesn't want to make may be made for him; "I think probably John won't be back," he says of Belushi, who with Dan Aykroyd has another career goint strong - The Blues Brothers, stars of a movie soon to be shot in Chicago.

"The success of 'Animal House' and The Blues Brothers has caused enormous confusion in John's life," Michaels says sympathetically. Belushi could not be reached for comment on that "Saturday Night" not only survived but prospered like crazy when the supposedly irreplaceable Chevy Chase left, so the show could probably adjust to life without Belushi. Yet it will be difficult to replace a presence this incendiary and amusing and to find another actor with a range that extends from Marlon Brando to Lou Costello and back.

Michaels thinks this was Bill Murray's big year on the show - he truly established himself as a member of the ensemble, and he thrived in sketch after sketch. But the exposure on Saturday night has, as with other members of the cast, led to other offers for Murray, in his case including the starring role in a film about "gonzo" journalist Hunter S. Thompson.

Michaels himself is producing a Broadway vehicle for Gilda Radner that will play the Wintergarden Theater for four or six weeks near the end of the summer. Tentatively titled "Live from New York," the show will also feature Don Novello, a writer on "Saturday Night" and the creator of the character of Father Guido Sarducci, the gossip columnist of the Vatican.

One of the problems with television is that people tend to regard it as only a step up to something better or merely as a vehicle for enhancing a career in some other medium. The beauty of "Saturday Night Live" has always been that it was the work of people for whom television seemed to mean everything. This, of course, is changing and the program is, inevitably, a victim of its own success.

"Do you know how terrible it is to be taken SERIOUSLY?" Michaels sulks. "Now everyone here is famous. I always felt, up to the middle of this season, that all that really mattered to anyone was the show. And then suddenly it all began to fragment. Now I find myself drawn to finding people who are more passionate about television."

This means pumping in new blood but now siphoning out old, Michaels says. "If our fifth year is going to be the last year of the show," and he implies that it is, "then I'd like to be more adventurous in the fifth year than we were in the first."

Not all the adventures are the knid he welcomes. This year's off-camera thrills included the fact that, one minute before air time, no one could find guest host Gary Busey. But - he turned up. The Chieftains, a merry band of music and mirth makers from the old country, had to be chased from bar to bar during rehearsals for the St. Patty's Day show.

Frank Zappa, a large professional zero on the pop cultural scene, proved the worst host in the show's history, as he mugged, forgot lines, and grinned in stupefying self-satisfaction at the camera. "I have a strong desire not to repeat his show," says Michaels.

And then there was that mythical medieval beast known as the one and only Milton Berle, who shocked everyone by living up to all his legends. "He walked all over me," says Michaels. "I was completely ineffective against him. I was mad at myself, that's all." Michaels skulked off after that show and didn't even attend the traditional weekly party. He's hard on himself and on the show, and so are the other people who write and perform it, and this helps keep it young and dangerous.

A few other plans went awry, including former staff writer Michaels O'Donoghue's program "Mr. Mike's Mondo Video," so dirty that the NBC censor said it would he aired only over his "dead body." Michaels thinks it may have a future as a midnight movie in theatrical distribution.

But for the most part, the year was another triumphant one for the "Saturday Night Live" show, one of the few TV programs in history both to dance on the cutting edge of hipness while at the same time hooting at the concept of a cutting edge of hipness. "Saturday Night Live" has helped spark a renaissance in American humor and satire, and it has done it in the best traditions of broadcasting - traditions that were largely dead and buried until Michaels and his pranksters revived them.

"There's no place in the world I'd rather be than at NBC right now," Michael says. "You have the feeling at least that you're in television here. I don't think this show could have come out of any other network." The gang got a graphic reminder of the kink of steps they were following when, in mid-season, workmen began dismantling a small suite near the control room that had been used for storage. They discovered that these were Arturo Toscanini's old dressing quarters, lavishly outfitted for the maestro when he conducted radio concerts form 8-H, the very studio where "Live" orginiates weekly.

Most of the furnishings in the apartment were discarded but the "Saturday Night" crowd got there in time to save a wall-sized mural of Toscanini conducting. It now has a place in the "Saturday Night" offices on the 17th floor. Toscanini would be proud - well, maybe not proud, but amused as hell. CAPTION: Picture 1, Bill Murray; Picture 2, John Belushi; Picture 3,Gilda Radner, of NBC's "Saturday Night Live."