Ten depressed and eight schizophrenic patients received synthetic human beta-endorphin infusions in a double-blind, placebo-controlled, crossover design. Physicians' and nurses' ratings and patients' self-ratings were used to measure behavioral change. Depressed patients improved significantly two to four hours after beta-endorphin treatment when compared with placebo treatment. There was no significant change in the schizophrenic patients as a group, although six of eight worsened after beta-endorphin treatment. No significant behavioral effects were observed during the infusions themselves or on postinfusion days.