"Mel Brooks' History of the World, Part I" is an entertaining mishmash of skits which finds Mel Brooks back in lively form, both for better and for worse. The only consistent thing about this burlesque miscellany, which incorporates skits about the Dawn of Man, Moses, the Roman Empire, the Spanish Inquisition and the French Revolution, is its inconsistency.

It may be easier to enjoy the movie, opening today at area theaters, if you decide to make allowances for the inconsistency in advance. The funning is rarely subtle. One winner is a great new gag about Hitler, always a sure source of comic inspiration to Brooks; but then there's a routine about Three Mile Island that is almost awesome in its lack of comic point.

Jokes about urination seem to proliferate.The film might be titled "Urination Through the Ages." Still, the initial sight gag on this obscene subject, presented as a primal scene of rivalry between artist and critic, is also difficult to resist. Could Brooks have envisioned it as a sneaky way of disarming his own critics?

To a considerable extent the funny stuff works in a laughing-in-spite-of-yourself way. For example, one might feel reluctant to credit Brooks with a stroke of comic genius for giving Cloris Leachman's Madame Defarge character a facial wart in a phallic shape. Nevertheless, there is something irresistably hilarious about the sight of Leachman scowling through revolutionary meetings with that ludicrous appendage twitching on her hairy chin.

In addition, the cast is loaded with familiar and semi-familiar comics and noteworthy bit players. The passing parade includes Sid Caesar, Howard Morris, Dom DeLuise, Madeline Kahn, Harvey Korman, Ron Carey, Gregory Hines, Shecky Greene, Beatrice Arthur, Charlie Callas, Paul Mazursky, Jack Riley, Art Metrano, Fritz Feld, Hugh Hefner, Pat McCormick, Sid Gould, Ronny Graham, John Hurt, Jackie Mason, Jack Carter, Jan Murray, Spike Milligan, John Hillerman, Sidney Lassick, John Gavin and even a flock of Playboy models, representing Brooks' conception of vestal virgins. The volume helps sustain the impression of a cheerful costume party even though the appearances may be fleeting.

Most of the segments are also fleeting. Brooks manages to sustain a comic plot only during the Roman segment, the longest but also snappiest portion of the show. His impersonation of Moses suggests that he might be sensational doing a definitive number on De Mille's "The Ten Commandments." Brooks appears in all five chapters, but he hasn't invented a character or thematic conceit to hold the parts together.

Brooks does some skating on thin ice, especially with an outrageous reenactment of The Last Supper, and a song-and-dance extravaganza on the Spanish Inquisition. In the latter, Brooks can't finesse the grotesqueness of a production number set in a torture chamber, with Orthodox Jewish victims doing patter recitative in the manner of Henry Higgins in "My Fair Lady." Ultimately, Brooks' own apprehension about the ugly implications seems to inspire him to cancel everything out by bringing on a chorus of bathing beauties for a parody of an old Esther Williams water ballet.

Despite these conspicuous slip-ups, "History" is certainly an abundant scrapbook of gratuitous laughs, and it represents a considerable improvement over "High Anxiety," where Brooks' comic instincts appeared to be flickering out.