The profoundly depressing aspect of "Two Brains," opening today at area theaters, is that a substantial portion of Steve Martin's public, which didn't exactly rally around his impressive work in "Dead Men Don't Wear Plaid" or the splendid "Pennies From Heaven," may prefer him in something relentlessly inane. If so, they can't ask for a sweller specimen, because "Two Brains" encases Martin's talents in a veritable bell jar of inanity.

Carl Reiner's direction is maddeningly frantic and disjointed from the outset, when the introductory scenes ricochet between Martin as a pompous brain surgeon, Dr. Hfuhruhurr (a progressively tiresome sort of running pronunciation gag throughout the picture), and Kathleen Turner as a mercenary tramp, Dolores Benedict, who are destined to become a marital disaster. The crosscutting is so choppy that the intended effect of fast, funny scene-setting degenerates into the unsightly illusion of a badly assembled preview trailer, desperately searching for "highlights."

Once the paths of the oblivious doc and the conniving slut cross--he accidentally hits her with his car--there's a fleeting interlude of satirical promise and purposefulness. Martin has a funny scene with a little girl who happens to witness the accident, entrusting her with elaborately detailed directions for calling an ambulance and alerting the hospital.

It's a relief to see that something works in a movie that suffers from such premature coordination problems.

The possibility obviously exists for a perversely satisfying spoof of inspirational romantic tearjerkers such as "Magnificent Obsession." Dolores supposedly requires all the remorseful doctor's surgical skills to save her life, but once she becomes Mrs. Hfuhruhurr, she makes his life a miserable, farcical hell of unrequited love and sexual frustration.

This potentially playable setup is quickly, decisively undermined by an excess of dumb, gratuitous gags and an ugly undertone of contempt for Kathleen Turner, who's really been had by Reiner. She labors conscientiously at slutty caricature without being able to anticipate how crude and degrading the cumulative effect will be.

To cite the most conspicuous bit, she's repeatedly obliged to suck on Martin's index finger in a mock-ravenous fashion--the emblem of Dolores' sexual deceitfulness.

What slowly riled me about this form of facetiousness is the belief that Reiner, and perhaps Martin as well, was getting a lot of sniggery, unsavory intramural satisfaction out of it. Perhaps I'm a cad to mistake their motives, but I kept receiving bad vibes along the boy-did-we-put-it-over-on-this-tootsie, and they became overpowering when Martin heaved Turner into a mud puddle with the line, "Into the mud, scum queen!"

The plot, thrown together by director Reiner and Martin in collaboration with George Gipe, goes off on a midcourse horror spoof tangent that vaguely explains the title but fails to salvage the show. On a trip to Vienna the doc makes the acquaintance of a mad colleague, played by David Warner, who has a collection of disembodied brains soaking in his lab jars, awaiting the surgical breakthrough that will permit him to insert them successfully into new bodies. A bit ahead of himself, Warner appears to acquire his experimental brains by committing, or at least collaborating in, mass murders.

Visiting the lab, Martin's lovelorn character strikes up a soulful conversation with a brain that calls itself Alice, and the rest of the movie expires strenuously while belaboring a doctor's dilemma that really presents no plausible problem in this context: How can Hfuhruhurr unite the mind of Alice with an equally savory body?

Every so often Martin transcends the stupidity of it all in a way that makes you crave a more intelligent exploitation of his talents. For example, he's quite funny when the doctor is articulating statements as eloquently fatuous as the following: "I can envision a day when the brains of brilliant men can be kept alive in the bodies of dead people!" When spoken, it sounds like a setup for some ironic comic payoff later in the movie, but if so, the follow-up evidently slipped everyone's absent mind.

There's also a moment when Martin goes into a simultaneous act of juggling and tap-dancing that serves as a brief reminder of what he's capable of. If Martin's fans really care about his development as a comic actor, they'll do him a great favor by evading this lethal stinker and insisting on early retirement for Carl Reiner THE MAN WITH TWO BRAINS

Directed by Carl Reiner; written by Carl Reiner, Steve Martin and George Gipe; director of photography, Michael Chapman; production designer, Polly Plat; edited by Bud Molin; music by Joel Goldsmith; produced by David V. Picker and William E. McEuen. Distributed by Warner Bros. Rated R. THE CAST Dr. Michael Hfuhruhurr....Steve Martin Dolores Benedict....Kathleen Turner Dr. Necessiter....David Warner Butler....Paul Benedict Dr. Pasteur....Richard Brestoff Realtor....James Cromwell Timon....George Furth Dr. Brandon....Peter Hobbs