This study was performed to examine the effect of naloxone on drinking behavior in a schizophrenic inpatient with psychosis, intermittent hyponatremia, and polydipsia (PIP syndrome). His body weight was checked five times daily, and the maximum and minimum weight gains during a day were chosen as an index of polydipsia. Both daily (0.6 mg) and repeated (0.6 mg for 6 days) injections of naloxone suppressed his weight gain significantly for 2 weeks. Withdrawal of the drug for 4 weeks resulted in weight gain recovering to control level. Thereafter, a second trial was performed to examine the long-term effect of this treatment. A daily naloxone (0.6 mg) injection series was performed once every 2 weeks for six series (12 weeks). This drug regimen also suppressed his weight gain in a continuous fashion. The study showed that naloxone seems to be a potential treatment for PIP syndrome and that endogenous opioid systems play a part in the compulsive drinking behavior of the PIP syndrome.