A mouse goes musical; Charlie and Algermon Musical with book and lyrics by David Rogers, music by Charles Strouse. Based on the novel, "Flowers for Algernon," by Daniel Keyes. Directed by Louis W. Scheeder. Musical director and conductor, Liza Redfield.

A husky young man and a small white mouse take the spotlight in a vaudevillesque song-and-dance routine. "Charlie and Algernon" came electrifyingly alive.

Those bright, brief moments supply the undoubted highlights of what its sponsors call "a very special musical," at the Helen Hayes Theater.

What makes "Charlie and Algernon" special and unusual is its subject matter. As those who read Daniel Keyes's novel, "Flowers for Algernon," or who saw Cliff Robertson in the film version will remember, the story concerns a mentally retarded young man who undergoes an experimental brain operation. (Algernon is the laboratory mouse who becomes his friend in the course of behavioral tests.)

Charlie quickly achieves prodigious mental capacities. Then, almost as quickly, he reverts to his former cheerfully childlike self -- but not before discovering a clue to why his treatment failed.

P. J. Benjamin makes an exceptionally appealing hero of Charlie, never losing sight of the identity within as he goes through the various stages of the experiment. The lovely Sandy Faison brings sensitivity and compassion to the role of the teacher and with whom he subsequently has a brief love affair.

The problem with "Charlie and Algernon" as stage entertainment is that the fragments through which it unfolds seldom achieve a commanding coherent force. There are snippets of dialogue and fragments of song. In addition to several romantic solos and duets, lyricist David Rogers and composer Charles Strouse have written a variety of numbers to suit the incidents in Charlie's tape-recorded journal: the classroom, the baker where he has worked, a disco for a spot of high life, and a traumatic flashback to a painful childhood.

Apart from the man-and-mouse title duo, "Reading" (in which Charlie sings and mimes his book reviews) proves yet another tour de force for Mr. Benjamin and a nice piece of comic invention.

If "Charlie and Algernon" seldom breaks out of its artifices, the adaptation is nevertheless consistently sympathetic, compassionate in attitude and treatment, decent and honest in its concerns. Director Louis W. Scheeder has handled a difficult subject with directness but also with the delicate touch it requires, responding to both the humor and pathos of the story, keeping sentimentality in hand.

The principal supporting players in a likable cast are Edward Earle and Rovert Sevra, who drily satirize a pair of white- coated medicos more concerned with the clinical outcome of their experiment than with its human implications.

Kate Edmund's coolly functional scenery accommodates the fluid action with a succession of mobile platforms plus slides and backdrops. The surrounding blackness creates a forbidding atmosphere which Hugh Lester's lighting has not been able wholly to relieve. The show has been attractively costumed by Jess Goldstein. Philip J. Lang orchestrated the melodic score, which is conducted by musical director Liza Redfield. Virginia Freeman choreographed the occasional dances.