The effect of naltrexone (0.5, 1.0, 1.5, and 2.0 mg/kg) on the frequency of self-injurious behavior (SIB) was investigated in three male adolescents. The frequency of total SIB was reduced significantly in all three subjects; dose-dependent decreases (at 0.5, 1.0, and 1.5 mg/kg) in SIB frequency were observed in the two mentally retarded subjects. These data suggest a role for opioid peptides in SIB.