The use of play therapy with mentally retarded children and adults is examined. The lack of research on the topic is noted, and information on psychoanalytically oriented play therapy approaches are reviewed. Application of play therapy to mentally retarded clients is explored in terms of two questions: (1) at what level do mentally retarded persons play? and (2) what kind of play offers therapeutic possibilities? Therapeutic benefits are ascribed to symbolic play, sand and water play, provision of opportunities for behavior change, and encouraging positive outcomes of the relationship with the therapist. Challenges to the therapist in working with mentally retarded clients are noted, including the client's past experiences with rejection. (CL)