The possible analgesic effect of melatonin was investigated in young male ICR mice. The formalin test which elicits typically 2 phases of pain response, the acute (first) phase and tonic (second) phase, was used. The test was performed in the late light period when the mice have been reported to be more sensitive to pain. Compared to control mice, no significant difference in nociceptive response was observed when melatonin was injected intraperitoneally at doses of 0.1, 5, and 20, mg/kg body weight. The combined effects of melatonin with diazepam and/or morphine, were also investigated. Melatonin, injected at 20 mg/kg 15 min before formalin test, significantly increased the antinociceptive response of diazepam (1 mg/kg) or morphine (5 mg/kg) in the second phase. In addition, when melatonin was given at 20 mg/kg together with diazepam and morphine, antinociceptive responses in both the first and second phase were increased. These data indicate the synergistic analgesia effect of melatonin with morphine and diazepam and suggest the possible involvement of melatonin as an adjunct medicine for pain patients.