Lots of things in life come back--things like rashes, mice behind the refrigerator and the check to the mortgage company.

Fortunately, so does "Monteith & Rand," a spiffy, thoroughly entertaining evening of comedy, much of it invented on the spot by John Monteith and Suzanne Rand. A pre-season hit last year at Arena's Kreeger Theater, the pair of comics returned last night to the scene of their criminally funny high jinks for a repeat engagement through Oct. 9.

They should have no difficulty duplicating their earlier success, although more than a few patrons may have trouble getting in. When it comes to Monteith and Rand, word spreads fast.

Here are the sort of people you're always trying to find when you're drawing up a cocktail party list--quick, attractive and enormously entertaining. They are capable of putting lamp shades on their heads, I suppose, but you can rest assured that they'd pick just the right moment to do it. She is a tall (taller than he is, in fact), animated brunet, with big blue eyes and a face that is wonderfully capable of registering hysteria and panic. He is a post-preppie with a growing resemblance to Paul Lynde, small dark eyes of the beady variety and a sly manner that could be described as laid-back, if laid-back wasn't one of the attitudes he enjoys sending up.

Their show is divided about equally between prewritten comic sketches and improvisations they concoct from suggestions hurled from the audience. In either case, they are decidedly greater than the sum of their engaging persons--true teammates bound together by some kind of comic ESP. He knows just how far to let her go before stepping in and taking over, just as she knows when he's about to hoist himself on his own petard. If she sometimes lets him, well, a little contrariness keeps a comic marriage fresh.

The written sketches deal with psychiatry, pickups, bars and pot-smokers. In one of the few repeats from the last go-round, Rand is a helpless stewardess for Allegheny Airlines at the controls of a runaway plane, and Monteith is the cool voice in the control tower guiding her in. (When he impugns the reputation of her airline, she huffs indignantly, "You know, we have changed our name." "You know," he retorts, "you're not fooling anyone.")

As for the improvised sections of the evening, they obey more or less the whimsies of the audience. One slot calls for a murder mystery--the locale, certain props and sound effects to be supplied by the spectators. Another has them playing out a given scene in sundry performance styles (among last night's requests: Tennessee Williams, soap opera, grand opera, Greek tragedy and Franc,ois Truffaut). Here they prove themselves masters of situational comedy. The lines that pop out of their fertile minds are not wildly funny per se. But the two are extraordinarily adroit at manipulating the context, which in turn makes the lines seem funny.

To say that one of the biggest laughs last night was provoked by the line "I'm sorry I'm late" is to say next to nothing. But consider the circumstances: She was in a submarine, waiting for orders. He was the skipper, who'd missed the boat. Suddenly he came swimming across the stage, knocked on the ship's hull, and then on a great whoosh of invisible water made his way through the hatch. Deadpanning his lame excuse at just the right pitch, Monteith got his laugh.

If "So you're a gay sky diver, huh?" or "If you can eat it, you can wear it" don't strike you as nuggets of hilarity, it really is because you have to be there, watching them mold and shape a sketch, as if it were silly putty.

Also held over from their last engagement is a segment devoted to instant songs. Audience members gave Rand "A stitch in time saves nine" to start with and "I like Ike" to end with, and she then proceeded to invent a blues in rhyme, as good as any on the charts today and twice as amusing. For his part, accompanying himself on the harmonica, Monteith elaborated a deft folk ditty beginning with "Do androids dream of electric sheep" and concluding triumphantly with "Eraserhead."

Midway through his song, he seemed momentarily to lose his bearings. (Who wouldn't with those guidelines?) "Don't panic," he assured the audience in a quick aside. "I know where I'm going." Indeed, he did.

Indeed, they do.

Straight to the funny bone.