Chronic electroconvulsive shock (ECS) has been previously reported to blunt the melatonin response to acute isoproterenol administration in rats. To assess whether electrically induced seizures are indeed required for the appearance of the blunted pineal response to isoproterenol, pineal and serum melatonin levels were measured after isoproterenol stimulation in rats treated with ECS (80 mA, 0.5 sec), subconvulsive shock (15 mA, 0.5 sec), or sham-ECS once per day at 11:30-12:00 h for 8 days. In ECS-treated rats, both pineal and serum melatonin levels after isoproterenol administration were significantly lower than those in sham-treated animals and in rats receiving subconvulsive shock. Moreover, as compared with sham treatment, chronic subconvulsive shock did not affect the melatonin response to isoproterenol. These data show that seizure activity is indeed required for the ECS-induced decrease in the pineal response to acute beta-adrenergic stimulation.