The Rockville Musical Theater has chosen for its spring production the durable but demanding musical, "Man of La Mancha. "The company does not always rise to the occasion musically, but succeeds in portraying the adventures of Don Quixote and his squire Sancho with considerable spirit.

J. Geoffrey Colton brings a strong presence to the title role, creating a highly credible and sympathetic Don Quixote. With restrained touches such as a slight-tilt of the head or a widening of the eyes, he suggests the confusion of Quixote's idealistic vision without sacrificing his dignity. Though his voice is not large, it is pleasant and intelligently used.

Jerry L. Payne, as Sancho, is an ideal complement to Colton. His pentrating, slightly nasal tone and simple manner underline the affectionate bewilderment with which the squire, equally dignified within his world, following his master.

Kathleen O'Toole makes a most attractive Aldonza, the servant whom Don Quixote perceives as his lady-love, conveying with intensily the transformation that Quixote's courtesies bring about in the illiterate peasant girl. Apart from a week rhythmic sense, O'Toole handles the vocal demands of her part creditably. One of the finest voices in the production is that of Carrie Kleemeyer, who portrays Quixote's niece, David Levin, as the padre, possesses an exceptionally mobile face and droll ways that make the listener forgive his inability to stay on pitch.

Set, staging and costumes are effective. The musical's frequent shifts in locale are quickly and imaginatively achieved. Among the particularly satisfactory moments are the lively fight scene in the inn and the dramatic entrance of the Knight of the Mirrors, who brings Quixote's wanderings to an end. Music director Ken Bissell provides solid support from the orchestra pit, setting a pace that moves the show along well.

Performances are at 8 p.m. at Rockville High School, 2100 Baltimore Road. For tickets, call 942-5005 after 6 p.m.