Melatonin (10 and 20 mg/kg) potentiated pentobarbitone (45 mg/kg) induced hypnosis; it (50 and 100 mg/kg) decreased both total and ambulatory activity; produced (10, 25 and 50 mg/kg) hypothermia and possessed (10-400 mg/kg) significant analgesic property both in tail-flick and acetic acid-induced writhing tests. Melatonin in low dose (10 mg/kg) significantly reduced the forced-swimming induced immobility period per se and also reserpine induced immobility. While the antinociceptive effect of melatonin was sensitive to reversal by naloxone (2 mg/kg), other CNS depressant profile resembled the effects of benzodiazepines.