The screen chemistry perfected by Fred Astaire and Ginger Rogers was once neatly summarized as follows: He gave her class, while she gave him sex appeal. A screwball kind of chemistry justifies the casting joke of "Oh! Heavenly Dog": Benji is improved by sounding like Chevy Chase, while Chevy Chase is improved by looking like Benji.

Now at area theaters, "Dog," is a blend of comic fantasy and murder mystery in which Chase and Benji share the role of a private eye, Benjamin Browning, who is reincarnated as a mutt in order to investigate his own homicide. A scruffy but appealing light entertainment, the movie owes its unexpected charm to the fact that comedian and dog seem to complement and humanize each other.

Chase's flair for wisecracking dialogue eliminates much of the sappiness that has softened Benji's image in the features and television specials devised for him by Joe Camp. Camp also directed this diverting aberration, from an absentminded script by himself and Rod Browing. In "Oh! Heavenly Dog" (the title pokes overdue ridicule at the genre of supernatural whimsy recently revived by "Oh, God!" and "Heaven Can Wait") Benji acquires the comic hostility and furtive sassiness that distinguished Francis, Cleo and Mr. Ed before him.

At the same time, Benji's natural warmth and vulnerability take the edge off Chevy Chase's unfortunate smugness, introducing welcome notes of humility and soulfulness.It becomes Chase to become the voice of a plucky little pooch.

Camp's clumsiness almost dooms the film to instant disaster. The early sequences, meant to establish the hero, inexplicably, as an American detective based in London and introduce him to the heroine (the lovely Jane Seymour, as a preposterously fashionable journalist), are so awkwardly directed and edited that the footage seems to be falling apart before your eyes. Each time there's a cut, you get that telltale impression that the director has switched to a different take as well as a different angle.

Camp evidently encountered location problems that were allowed to confuse the exposition. The six-month British quarantine on animals made it uneconomical to shoot the principal footage with Benji in the ostensible setting, so Montreal doubles for London most of the time. The filmmakers often misplae the characters while switching scenes. Seymour appears to drive directly from Paris to London on one occasion. The principal suspect is left hidden in a closet at the scene of the crime and then forgotten when the scene shifts. Benji escapes from certain confined situations without explanation, although the whole point of the movie would appear to be showing us how he contrives to get out of all the tight spots invented to frustrate or threaten a canine shamus.

The movie begins to display a flicker of wit when Chase, murdered soon after discovering the corpse of a woman he'd been hired to shadow, wakes up in a congested, computerized eternity, where bureaucrats inform him that he'll be expected to crack the case, to prove that he's better material than the records indicate, but will have to return in the best available form. Rather than haggle, he accepts a dog. It beats going back as a rat.

The funny, ingratiating side of the film shows itself decisively when Benji does his first bit of nosing around. Entering the apartment that was the scene of the murder, he begins looking for clues. The comic illusion created by combining Chase's tart commentary with Benji's diligent efforts to gather evidence (and simply get around town) on four diminutive legs proves captivating.

It comes as a particularly delightful shock to hear Benji utter a swear word after he finds a possibly significant telephone number but can't manipulate the dial with his paws. Undeterred, the hero endears himself even more by grabbing a pencil with his teeth and using it to dial the number. When the party answers, Benji, forgetting his limitations, tries to converse, "Woof, woof" is all that comes out.

The humor implicit in the hero's transformation and several situations which dramatize it effectively manage to save the initially floundering show. Enjoyable as it becomes, the prototype realizes only a fraction of the beguiling and hilariously suggestive possibilities.

Reunited with the heroine, the hero is placed at an apparent romantic disadvantage that pays humorous dividends. Content to be discreetly risque, "Oh! Heavenly Dog" shows the hero making the best of a humilitating situation. He resents being unable to court the heroine properly but takes sneaky compensatory pleasure out of playful voyeurism and the sabotage of potential rivals. When one obnoxious suitor, a conceited artist, tries to make nice with Seymour, Banji-Chase makes an ingenious pest of himself.

Camp and Browing also create a funny rapport between the leads by playing on their inability to communicate directly. "Serves you right," Seymour says upon hearing the dog sneeze, the morning after he has jumped into her bath. "No regrets," Chase confides.

One marvelous suspenseful situation illustrates how satisfying the premise could be if engineered for maximum imaginative fun. Following a lead, Benji goes to a post office and finds a crucial piece of evidence in one of the boxes. The villain arrives soon after, setting up a remarkable confrontation. From the outside of the glass box he sees the gallant dog on the other side, reaching for the envelope inside with his paw. While the villain desperately struggles to work the combination lock that will open his side of the box, Benji struggles to extricate the envelope, get it in his teeth and make a getaway.

It's a shame top-flight directors seldom practice their skills on movies aimed at juvenile audiences. It would be gratifying to see the fanciful premise of "Dog" sustained as cleverly and attractively as Brian De Palma sustains an equally transparent plot for grownups in "Dressed to Kill."