THEY HAVE only 45 minutes between shows, and the conversation flies:

"What's the hook?"

"Maybe it's just four guys lamenting what's become of women."

"We've definitely got to stay away from the gay thing. No mincing around."

"We're really gonna get nailed on that."

Eventually, spontaneously, a joke is born.

Over the years, the scene has repeated itself many times at Chicago's Second City comedy theater. Seated around the trestle desk of producer Bernard Sahlins, players Tim Kazurinsky, Bruce Jarchow, George Wendt, Danny Breen and stage manager Larry Perkins hammer out a new skit, a barbershop-quartet jab at today's Hollywooded hair-cutteries.

The gag is part of the company's new revue, marking Second City's 20th year as a showcase and training ground for the country's hottest improvisational comedy talent.

Once the faces belonged to John Belushi and Bill Murray and Gilda Radner. And before them Mike Nichols and Elaine May, Alan Arkin and Barbara Harris. All are Second City veterans, and the atmosphere is heavy with psychic residue.

"It's not like this all the time," says Wendt. "We don't always work this hard. But we've got to live up to the myth."

And everyone wants to know: With Belushi and Aykroyd gone from "Saturday Night Live," who's next?

"Us, of course," says the bespectacled Breen, pointing to himself and Wendt.Quiet laughter ensues. It is generally agreed here that "Saturday Night Live," despite its undiminished popularity, is not the stellar production of old.

Bernard Sahlins, the balding owner and producer of The Second City, is seated behind his desk lighting a big cigar, discussing the state of humor in America. Outside it is a bright Chicago fall morning, and below Sahlins' window Second City's national touring group is loading the van for a three-day trip through Wisconsin and Iowa.

Meanwhile, the phone rings merrily off the hook. Some shows at the club -- with an audience of 300 at $5 a ticket -- have been booked as far in advance as next March.

Only a few years ago, Sahlins and his staff were jammed into quarters that have since become the ticket office.But that was before Second City became a humor conglomerate -- overseen by the parent company in Chicago -- which includes two companies in Toronto (Dan and Peter Aykroyd are veterans), another company in Dundee Ill., the national touring company playing to nightclubs and college campuses throughout the Eastern United States, a contract for an NBC television pilot and million-dollar production deals.

Second City's television show, going into its third season, is seen on 67 stations around the country, at last count. Executive producer Jack Rhodes in Los Angeles says the syndicate is adding new clients almost every week.

Produced in Toronto largely because indirect government assistance helps keep costs to a minimum, the TV show was temporarily cancelled in response, some sources said, to increased pay demands. Second City's Toronto owner, Andrew Alexander, says production should begin again soon. But with more than 50 episodes already in the can, the show is expected to continue its successful ways, cancellation or no.

Within the last year, its audience share in Chicago has risen from 13 to 26 percent; 7 to 13 percent in Los Angeles and 15 to 27 percent in Rochester, N.Y. In areas where it follows "Saturday Night Live," "Second City Television" has proved itself especially popular among late-night audiences. In New York, Chicago, Seattle and Portland, said Rhodes, the show is rated first in its time slot. In Washington, where it is carried by WDCA-TV, two network affiliates have made overtures toward obtaining syndication, Rhodes said.

Second City has come a long way since taking over a converted Chinese laundry on Chicago's north side. For two decades it has survived as a comedic force and training ground thanks largely to the comedic sense and munificence of Bernard Sahlins.

"I find that class for class, intellect for intellect and vocation for vocation, people are less interested in issues than they were 10 or 15 years ago," Sahlins said. "The humor they respond to today is humor that derives from the basic ridiculousness of life. I was surprised at the lack of outrage there was about the oil profits. People are definitely more submissive."

Sahlins has the last word on what will and what will not appear at Second City. These days it is a blend of zany and classic satire, some pokes at suburbanites and a few at television and Chicago politics. What you will not find, he says, is the kind of gratuitous bad taste that in recent years has crept into other acts, most visibly "Saturday Night Live."

"Bad taste is part of a certain segment of our humor. If there is a bizarre or corrupt element, then it is reflecting people's uncertainty and doubt. The shock of Vietnam and seeing atrocities beamed into our living rooms every night may have changed our vocabulary." But "the humor of behavior is coming back."

Leafing through the latest issue of "Variety," he finds the movies "10" and "Breaking Away" illustrating that point: both very human, both very successful.

On Saturday night, the resident company has finished its second show. This is followed by improvisations, spontaneous skits born of ideas offered by cast members and the audience. Tonight they are honing seven numbers for the new revue.

On the previous Tuesday night, a member of the audience had suggested Jesse Jackson as the butt of one skit. The resulting act portrays a team of Russian KGB agents training to infiltrate the U.S. as the Soviet idea of normal Americans. Jackson is their main operative.

The linebacker-sized Wendt is dressed as a fashion model in leopard skin coat and platinum wig. Kazurinsky is a kid's-show host wearing a propellor hat and hoola hoop. Breen is a redneck from Arkansas who sells chain saws and is married to a 16-year-old with no teeth.

"Hello. What is your sign?"

"Hello. Here's five dollars."

"Oh! Is that a tip? Thank you very much."

A crack-up.

Behind stage afterwards, the crew, including players Nancy McCabe Kelly, Mary Gross, the soon-to-depart Larry Coven and director Del Close, gather around a table littered with empty beer glasses and Perrier bottles. They discuss dead spots in the act.

When the exchange crescendos into a free-for-all buzz, a spontaneous and shattering "Focus!" from Perkins brings immediate silence.

With no professional writers to speak of, Second City's material is developed in front of late-night audiences and in the sessions behind stage. The revue is written down only some weeks after the show has already opened, transcribed from audio tapes.

"I think that's when 'Saturday Night' started to go," says Close later in the dark and deserted theater, "when they started using writers to come up with the material. Those people were all trained in improvisation and when they stopped doing that the show lost something."

Close, a former stand-up who helped start Second City Television two years ago, is credited with keeping the show's rapid-fire format right on target. ("When Robin Williams came here," Perkins says, "he was overwhelmed. He said, 'I can't believe I'm really meeting Del Close.'")

"There are two watershed marks in contemporary comedy," Close says. "The first was Shelly Berman considering a glass of buttermilk. The second was Danny Aykroyd putting the fish in the blender.

"A lot of the stuff 'Saturday Night Live' was doing came out of National Lampoon, comedy based on outrage and bad taste. Murray, Belushi and Aykroyd, those guys are the brotherhood of rage. They all did some pretty weird things."

By 5:30 Sunday morning, after a long session in a Wells Street bar, stage manager Larry Perkins is ready for breakfast. He takes a taxi to a North Side deli and orders steak and eggs.

"These people are all good people," he says, forking rib-eye into a bearded mouth. "They all bring something to the show. They're fast. But they all know they won't be here forever. They'll go on to other things."