Rats injected with the opiate antagonist, naloxone hydrochloride (10 mg/kg), 15 min prior to sessions in which they were given free food on a fixed time 75-sec schedule, displayed retarded acquisition of schedule-induced polydipsia relative to vehicle-injected subjects. Rats injected with naloxone after schedule-induced polydipsia had been acquired were unaffected, i.e., they continued to drink at control levels. Given that schedule-induced polydipsia has been considered non-opioid in nature, because of previous reports of its insensitivity to naloxone, the present report of differential effects of naloxone on the acquisition and maintenance of schedule-induced polydipsia suggests that some modification of this conclusion is necessary. Possible alternative mechanisms for these differential effects are discussed.