Chicago is only so big. Sure, it's a pretty big place, but just not big enough for the two of them. The birthplace of improvisation had room for only one improvisation troupe. And since Second City -- home of such comedians as Joan Rivers, Alan Arkin, Bill Murray and Gilda Radner -- was pretty well entrenched, Chicago City Limits had to hit the road.

It did. To New York.

The 13-year-old comedy company now has found a safe and successful haven on the Upper East Side of Manhattan, in a church theater called Jan Hus. It has workshops there and performs six nights a week -- with five comedians on stage and a piano player. About 60 percent of the act is pure improv, the other 40 a mix of rehearsed skits and satire.

The artistic form known as improvisation dates back "hundreds and thousands of years," says director Linda Gellman, "but improvisation as a theater form is considered to have started in Chicago in the 1960s, at the University of Chicago."

The goal of improv is to come up with a comedy scene instantaneously on a subject provided by an outside source.

"We ask for a suggestion, like a problem, or a piece of news, a headline, or something that gets you down, or an opening line of dialogue," says Gellman, "and with that, we create a character or a situation or a song. We do it on the spot. And the audience sees a scene or a character created right before them."

"The greatest compliment," she adds, "is when someone comes up after a really hot show and says, 'That was really great, but you can tell me. You didn't make that up right there.' "

One of the comedy company's favorite routines is the "One Word Story." Each comedian says one word, one right after the other, very quickly, supposedly creating a story as they go. If it's not going well, the audience lets them know, calling out and booing. If it goes right, though, they get chants and cheers.

"It's kind of like a mental workout," says Gellman, "but if it works, hopefully, you come away with a little gem."

Chicago City Limits makes its Washington debut Thursday night at 7 at the National Museum of Natural History's Baird Auditorium. Tickets are $14 for Resident Associate program members and $19 for non-members. For information, call 357-3030.