The objective of the present study was to assess whether the anticonvulsant activity of melatonin displays diurnal variability in hamsters. Convulsions were induced by administering 3-mercaptopropionic acid (3-MP). There was a significant diurnal variation in 3-MP-induced convulsions, hamsters being more prone to exhibit seizures during the night than during the day. Melatonin (50 mg/kg i.p.) had a maximal anticonvulsive effect in the early evening (20:00 h). The administration at 20:00 h of the central-type benzodiazepine antagonist, Ro 15-1788, although unable by itself to modify seizure threshold, blunted the anticonvulsant response to melatonin. The results indicate that the time-dependent anticonvulsant activity of melatonin is sensitive to central-type benzodiazepine antagonism.