Monteith and Rand, the congenial comedy team that set up shop in Arena Stage's Kreeger Theater on Tuesday for a four-week run, would just as soon keep their best lines for the paying public. As the warning in the program puts it: "Revealing excessive amounts of the material in this show to anyone who has not seen it may be injurious to Monteith & Rand's health."

Far be it from me to violate their wishes. If we can't respect a punch line these days, what can we respect? I assume, however, that it is okay by Hoyle to describe some of the sounds emanating from the Kreeger.

Since Monteith and Rand are a very funny pair, the most obvious noise is produced by the audience, which begins laughing early on, continues laughing until the final blackout and generally conducts itself like a string of firecrackers on the Fourth of July.

Since Monteith and Rand are also improvisational comics who ask the audience for place names, lines, movie styles, pet peeves and the like, which they then proceed to incorporate into sketches born on a moment's notice, the Kreeger is periodically flooded with babel from the balcony and orchestra. (In the interest of sociology, I report that their requests for a pet peeve on opening night elicited, among others, "burping" and "mass murderers." You figure this country out.) It's a clear case of everyone trying to get into the act, but when the act is as likable as Monteith and Rand's, the impulse is understandable.

The most distinctive sound of the evening, though, is really the whir and click of two very fertile minds at work. Dean Acheson called one of his chronicles "Present at the Creation," and however thrilling that inside track may be in politics, it is doubly so in show business.

We may be watching Monteith and Rand traipse through mysterious Tierra del Fuego in search of a certain Mrs. Gonzales' dead husband, seemingly pelted to death with Chicken McNuggets -- which is where one batch of audience suggestions led them. Or we may be watching them prepare a State Dinner at the White House, as Hitchcock, Ingmar Bergman, Stephen Sondheim and Shakespeare might have written the scene. What really has us entranced is the mad machinery inside their heads -- all the crazy coils and springs and interlocking levers that somehow manage to produce a sudden moment of fine nonsense. Improvisation of this quality is the theatrical equivalent of a brain scan.

It has to be their brains we're looking at, for while they are both pleasantly featured creatures, neither is exactly prepossessing. Monteith (first name John) is the shorter of the two, given the excessive high heels that Rand (Suzanne) favors as footgear. He also has a face a bit like a warm pudding and a downward cast to his mouth that suggests Paul Lynde as a post-preppie. Her most impressive physical attribute is a mane of auburn hair -- a perfect prop for the frazzled states she often finds herself in -- and a very stretchable mouth. Still, they don't exactly appear a natural comic pairing, like Mutt and Jeff or Laurel and Hardy.

Not, that is, until they get down to business, at which time it becomes obvious that her flamboyant nervousness is a grand counterpoint to his slier, more reserved airs. She tends to totter on the edge of panic. He stands off a few steps and eyes it wryly. If she's an incipient rag doll, he's a potential teddy bear. In one of the written sketches that alternate with the improvisations, she plays a desperately helpless stewardess at the controls of a runaway airplane, trying to land at National. He's the cool voice in the control tower guiding her down. ("Allegheny Airlines?" he asks knowingly.) When he ends up in a dither, which he does occasionally, it's usually because she's dragged him there. Notice she's the one who gets stoned, swats a fly, then, remorseful, tries to revive it.

If truth be told, the written material is a notch or two below the improvisations. It's not tipping things to say the preset routines involve the confessional, sex changes, Brooke Shields, movie critics, shrinks and an X-rated photo booth where four quarters gets you four poses with an inflatable doll. Some of it seems like filler, however, between the real bursts of fun, which occur when the duo ventures into uncharted territory.

One of their on-the-spot segments, for example, is devoted to instant songwriting. Using "Coke is it" as her opening line and "You're soaking in it, Madge" as the capper, Rand concocted a cripplingly funny blues number -- in rhyme, no less. For his part, Monteith resorted to the country mode and managed an equally zesty saga of the open road that dragged "Gone With the Wind" and "Apocalypse Now" into the lyrics.

At moments like these, Monteith and Rand are true daredevils of comedy. They take their lives into their hands by stepping into contraptions marked for sure disaster. Then, boldly and triumphantly, they proceed to maneuver them across the high wire.

MONTEITH AND RAND. With John Monteith and Suzanne Rand; assisted by Bill Russell; costumes, Donald Brooks. At Arena's Kreeger Theater through Jan. 2.