Neither fish nor fowl but definitely a musical that could be called ''Hare'' is the attraction that opened last night at the Gershwin Theater under the title ''Bugs Bunny on Broadway.''
Coinciding with the 50th birthday of the rowdy rabbit of animated cartoon fame, the show is an unusual mingling of movies and music, primarily consisting of an assortment of ''Looney Tunes'' shorts and segments unreeled to the accompaniment of a 40-piece orchestra seated onstage before the screen.
From time to time, however, the emphasis is solely on orchestral performance: a war horse like Liszt's Hungarian Rhapsody No. 2 or a salute to Carl W. Stalling, who, with Milt Franklyn, mingled his own brand of genius with that of Rossini, Wagner, Donizetti, Suppe and others to provide so much of the music that accentuates the antic air of the cartoons.
So the audience spends no little time watching the equally animated conductor George Daugherty and his heavily miked Warner Brothers Symphony Orchestra both on the stage and as projected on the screen by video. The chief joy of the event is the opportunity to see the cartoons, directed by such legendary animators as Chuck Jones and Friz Freleng, who received a standing ovation from the audience at the preview on Wednesday evening when they were introduced onstage by Mr. Daugherty, who is also the producer of the show.
Mostly built around music, the cartoons included the ''Rabbit of Seville,'' a hilarious reworking of Rossini, in which Bugs, pursued by a shotgun-wielding hunter in the person of Elmer Fudd, survives a close shave with death to beard his foe; ''Long-Haired Hare,'' in which Bugs, disguised at one point as a white-maned conductor referred to as Leopold, literally brings down the house on a pompous opera star; ''Rhapsody Rabbit,'' keyboard chaos set to Liszt, and ''What's Opera, Doc,'' a Wagnerian sendup in which the horn-helmeted, spear-carrying Fudd sings, ''Kill the wabbit.''
For adults, there are sly digs here and there at Disney's ''Fantasia'' and for grown-ups and children alike, ''Bugs Bunny on Broadway'' affords the opportunity to appreciate the gifts of the artists who imbued the Warner Brothers' ''Looney Tunes'' gang with their anarchic spirit. But the amplification saps the orchestral music of the brilliance of live performance, and the clearly visible conductor's monitor shows that on the large screen, the animated films are also drained of color and sharpness.
Still, the packed house was clearly reluctant at the final curtain to accept the classic ''Looney Tunes'' farewell: ''That's all, folks.''