"Zelig," the shape-changer they called the human chameleon, makes "Sybil" look sane. In a face-off with "Eve," there's simply no contest. The character splits infinitely.

Leonard Zelig, nut case, all the rage in the craze days of the Jazz Age, was a fad like flagpole-sitting or bathtub-gin fizzing, a pop hero created by Movietone News for the thrill-hungry public.

Zelig is also a figment of the imagination, the subject of Woody Allen's new pseudo- documentary, a cerebral comedy made with wizardry and wile. The film, a flawless unity of old and new footage, flickering and purposefully aged, parodies serious documentaries and portrays a time when the world got the sillies.

The extraordinary psychoanalytic journey of this fictional figure is a brillant conceit honed and polished in top secret by the director, writer, actor and his minions. Allen, mellow with a touch of tang, plays Zelig, a man whose need for acceptance leaves him without an identity. He's always changing into something more comfortable. He turns black or yellow, grows fat or thin, becomes a rabbi or an Indian chief, depending on whom or what he's with at the time. Except females.

His multiplicity attracts Mia Farrow, who's superb as Dr. Eudora Fletcher, his no- nonsense psychoanalyst. Alas, when Zelig is with the doctor he becomes a doctor, and Fletcher is at first unable to convince him that he's her patient. Eventually she succeeds by pretending to be his patient and the happy, now affianced, couple are hailed in the newsreels. In one, Zelig advises, "Kids, you've got to be yourselves."

The bogus reels and interviews, some in color, some in black-and-white, are wonderfully natural, part of 75 hours of new and 30 hours of stock footage winnowed to about 80 minutes of finished film. It is seamless, high- tech weft and warp, interwoven by an adept circle of miracle-workers, including editor Susan E. Morse and photographer Gordon Willis.

Old lenses and optical devices reproduce grainy, quavery shots of the cast that were superimposed over the old stuff. Allen can be seen coming up to bat behind the Babe, for instance, or hobnobbing with Hitler. Allen's authentic-looking film within the film, called "The Changing Man," 1935, illustrates Zelig's cure, remission and various disappearances. Old photographs, screaming headlines and acoustical verit,e complete the illusion.

Beyond the technical feat, Allen has a sweet little story to tell, an update on the state of his psyche since "A Midsummer Night's Sex Comedy." The auteur's audiences have always enjoyed his movies as a form of group therapy, and "Zelig" -- German for soul -- doesn't disappoint. The comedian, little in evidence here, seems to be giving up sex for spirituality, arriving past midlife with Farrow securely in tow.

As one fictitious expert says, Zelig just wanted to be loved -- a comment so simple- minded it's funny. Another recalls, on a sillier scale, that Cole Porter was fascinated with the guy. He wanted to write, "You're the tops / You're Leonard Zelig." But he couldn't think of anything to rhyme with Zelig. The taped interviews also form an amusing parallel to Warren Beatty's "Reds."

Amusing, remember, not hilarious. "Zelig" is not a linear laugh fest. It's usual for Allen only in that it's unusual. While critics swoon and devot,ees marvel, others who expect to be entertained in a traditional fashion might be bored after waiting patiently for the film to go beyond technique and get on with the customary storytelling.

For the right viewer, it's a film to be savored and pondered. As the author of "Interpreting Zelig" points out, Freudians, Jungians, actors, all types of interpreters loved the human chameleon because they could interpret him anyway they wished. Of course, the same is true of movie mavens, which is why this review is so damn long. Splitting personalities is as time-consuming as splitting hairs. ZELIG -- At the Avalon.