A mentally retarded patient who developed polydipsia and hyponatremia following the long-term use of neuroleptics is described. Before the investigation, he was treated with a combination of five drugs : diazepam, propericiazine, carbamazepine, lithium carbonate and trihexyphenidyl. The treatment was switched to diazepam alone; to propericiazine, trihexyphenidyl and diazepam; to carbamazepine and diazepam; and finally to lithium carbonate with diazepam. Under the five drug treatment protocols, his weight gains during the day were monitored, every day as an index of the water he drank in the day time. The combination of the five drugs caused the greatest weight differences per day and the other drug regimens, except diazepam alone, seemed to increase the weight differences to some extent. These findings indicate that a drug combination such as the neuroleptics, lithium and carbamazepine used for emotional stabilization must be carefully monitored to avoid the induction of compulsive drinking.