Chronic electroconvulsive shock (ECS) has been shown to induce a downregulation of beta 1-adrenergic receptors in the rat cerebral cortex. Because the secretion of melatonin in the pineal gland is regulated primarily by beta 1-adrenoceptors, in the present study we investigated the effect of chronic administration of ECS on pineal beta-adrenergic responsiveness to isoproterenol. To this purpose, young adult male rats received once daily for 8 days ECS (80 mA, 0.5 s) or sham ECS. On the day after the last ECS or sham treatment, they were injected with isoproterenol hydrochloride (1 mg/kg SC) or volume-matched saline at 1600 h. Two hours later they were killed by decapitation. Results showed that the isoproterenol-induced increase in the pineal melatonin content was blunted in rats treated with ECS as compared to sham-treated animals (shock x drug interaction = p < 0.01). These data indicate that chronic ECS treatment affects beta 1 receptor-mediated melatonin production in the pineal gland. Further studies need to elucidate whether the blunted melatonin response to isoproterenol in ECS-treated rats is due to a downregulation of pinealocyte beta-adrenergic receptors.