In "The Main Event," a successful perfume manufacturer is victimized by an embezzler who leaves her with a single source of income: an unsuccessful prizefighter who hasn't boxed in years.

Desperate, she nags him into a comeback. It turns promotable when the pair end up bickering in public after her ignorance costs him a bout by disqualification. Eventually, the put-upon pug and the brassy boss-lady fall in love and vow to exchange wedding rings for the boxing ring.

This premise looks remarkably unappealing on paper, and doesn't improve in the playing. New romantic comedies seem to be degenerating at the moment, and "The Main Event" is nothing to rave about.

As the perfume tycoon, Barbra Streisand bobs, weaves and jabbers pugnaciously under the incongruously classy name Hillary kramer. It seems one of the funnier unintentional jokes in the lackluster script attributed to Gail Parent and Andrew Smith. A name that would suit Streisand in "The Main Event" is Rusty. Maybe Rusty Ringlets. (Those Harpo Marx hairdos have been clowned up even more by a rust-hued rinse that creates the impression of red fright wigs.)

Streisand's personality and technique have grown rusty, too. Many contemporary stars grow too wealthy or complacent to act as often as they should, but it's probably better to sit tight than jump into a vehicle as unchallenging as this one.

One keeps hearing that Streisand intends to direct her next movie, whatever it is. Surely this step will be a mere formality, calculated to enhance her self-esteen yet unlikely to improve her taste or judgment.

Streisand evidently assumed authority on the set of "A Star Is Born," a prospect foreseen by several prominent directors who turned the project down, leaving Frank Pierson, a successful screenwriter with unfulfilled directing ambitions, to take nominal command - to his well-publicized grief.

Streisand looks undirectable now, but this impression is no more pronounced in "The Main Event" than it was in her three preceding pictures - "A Star Is Born," "For Pete's Sake" and "Funny Lady." She was an eccentric talent from the outset, but she's deteriorating. In certain early vehicles - "Hello, Dolly! or "On a Clear Day You Can See Forever" - she seemed triumph over miscasting. Now she exercises the right to miscast herself.

"The Main Event" may not have been a First Choice script, but no one compelled Streisand to undertake it. Her production company was in charge, and according to people close to the shooting, "She and Jon [boy-friend Jon Peters] just totally took over."

Streisand's role turns into a miserable variation on the farcical characterization she did brilliantly in "The Owl and the Pussycat." But she's too old for that prototype any more and also too domineering to seem appealingly brash.

No longer an aspiring kid's defense mechanism, that brashness has evolved into celebrity self-assertion. The emotional vulnerability that made Streisand a romantically stirring heroine in the widely varied contexts of "Funny Girl," "The Owl and the Pussycat," "They Way We Were" and "Up the Sandbox' has disappeared, leaving only a big-name content to dominate what appear to be thinly disguised home movies.

Although she's initially identified as a capable, inventive businesswoman, the heroine quickly reverts to scatterbrained hoyden, a type only marginally more intelligent, sophisticated and - here's the rub! - desirable than the erstwhile boxer's steady girl, a foulmouthed tart who monopolizes all the aggressive jokes preoccupied with genitalia. Why should Streisand scorn an apparent opportunity to move up the social ladder into what was once Rosalind Russell territory? She evidently finds it more comfortable to act like an overage groupie, but it is not a becoming preference.

The footage grows top-and bottom-heavy with images of Streisand thrusting her bosom and behind at the camera. It's curiously off-putting, not nearly as inviting as the sight of her sharp jaw as she repeatedly juts it out at Ryan O'Neal as the boxer, asking for it as no leading lady has since Cybill Shepherd in "At Long Last Love."

Although Streisand goes out of her way to demonstrate how supple she is during an early expository scene setat a women's exercise class, the display does not lend itself to prolonged contemplation. And the T-shirts and peekaboo shorts don't do the same thing for her that they do for Susan Anton, to take an extreme photogenic case.

Moreover, the whole libidinous coloration imposed on her character in "The Main Event" seems tiresome and fundamentally anti-romantic. The essence of the turn-off is expressed in a speech she's given when she and O'Neal finally snuggle into the sack for the first time at his training camp.At first this intimacy looks like a blessing, since the movie has squandered a quarter hour or so on jokes about the lewd possibilities of the heroine residing, implausibly, in the camp's dorm with an all-male entourage.

The relief is short-lived. "I guess it's hard being in bed with me and not touching me," the heroine hints. "I don't want to drive you crazy or anything." Unfortunately, she's not being facetious. This come-on is considered irresistibly stimulating rather than excruciating. Heaven knows how much time and effort went into writing and improvising this extended, would-be conclusive romantic encounter, but it's an impotent bummer from the opening overture. Bob Hope and Trigger enacted a far more amusing bed scene in "Son of Paleface" when they did little but try to hog the covers.

O'Neal has also looked fresher and been in sharper form than he is in "The Main Event," but he's a more appealing figure than Streisand. After all, his character has to take the lumps as a result of the improbable business partnership contrived by the screenwriters. It becomes O'Neal to seem a trifle battered and seedy. He also contributes the best spontaneous physical gag in the show, a fall (while leaning back in a chair) that Chevy Chase might envy.

There's a felicitous bit of business or line of dialogue every so often - particularly the introduction given O'Neal's character by a ring announcer: "And in this corner, the fighting driving instructor: Eddie 'Kid Natural' Scanlon!" James Gregory strides casually into the continuity in the role of a Runyonesque fight promoter and reminds you what an adroit style of comic acting once looked and sounded like.

As much as I prefer fast-talking comics and once thrilled to Streisand's verbal tempo, I don't think her acceleration enhances the undistinguished chitchat and disputation in "The Main Event." The effect is similar to Glenda Jackson's rapid, clipped line readings in "Lost and Found." You feel as if both actresses are rushing through the dialogue in hopes of obscuring its lack of wit and polish.

Howard Zieff's direction and Mario Tosi's cinematography also look rushed and overemphatic. The shabbiness isn't characteristic of their earlier efforts, including their work together on "Hearts of the West." CAPTION: Picture 1, Barbra Streisand and Ryan O'Neal in "The Main Event."; Picture 2, Barbra Streisand in "The Main Event."