Plasma haloperidol levels were monitored in three schizophrenic patients when carbamazepine was either added or discontinued. The percent decrease in plasma haloperidol levels due to concomitant carbamazepine therapy was between 59% and 61%. The effects of carbamazepine on plasma haloperidol levels were noted to occur in 2 to 3 weeks. Although no adverse effects occurred in the patients during therapy, careful monitoring of clinical symptoms and plasma haloperidol levels is recommended.