In a study of behavior modification two Down's syndrome preschool children, the first a 5-year-old boy with autistic behavior patterns and the second a 4-year-old girl whose behavior problem was to reject other children, were examined. The first S was engaged in ball catching activities with a teacher with positive reinforcement (playing the S's favorite music) for returning the ball to the space between the teacher's legs. The activities lasted for a 1-year period, at which time the S was actively playing catch with his teacher. Among conclusions were that the S's autistic behavior patterns were remarkably modified and the reinforcement function was verified. The second S was engaged in trampoline play with a teacher and the first S. Positive reinforcement (praise, physical contact) was given for target behaviors (such as laughing, giving the first S a toy, or touching him). Results after 20 sessions showed that the second S's approach behavior had changed considerably. (Tables of S responses are included.) (PHR)