A clinical trial of naloxone to treat head banging behavior in a severely mentally retarded 8-year-old boy is reported. A double blind procedure was employed to concurrently contrast effects of two dosages of naloxone with saline solution placebo. No quantitatively measurable change in response rate occurred compared either with baseline or placebo. However a qualitative reduction in intensity of response was noted clinically. Implications for the use of β-endorphine therapy for self-injurious behavior was discussed.