In order to develop melatonin (MT) as a potential new drug for the treatment of diseases with inflammation, pain, and abnormal immune responses, the effects and mechanisms of MT on inflammation, immunoregulation, and nociception were studied systematically. MT (40-1 60 mg/kg, ip) had significant analgesic effects in the hot-plate, writhing, and tail-flick models, with a marked dose-and time-dependence. The onset of its analgesia about 30-60 min after ip, was slower than that of pethidine, but the duration was longer (about 1.5-2 h). The analgesia was also induced by icv MT (0.25 mg/kg) injection. A lower dose of MT (10 mg/kg) could enhance the analgesic effect of pethidine, which was blocked by naloxone (1 0 mg/kg). MT (100 mg/kg, ip) decreased the content of beta-endorphin in the hypothalamus and pituitary. The analgesia of MT could be attenuated by pretreatment with reserpine (30 mg/kg, ip) or phentolamine (10 mg/kg, ip). CaCI2 (230 mg/kg) could antagonize the analgesia of MT. EGTA and verapamil had opposite effects to CaCI2. No tolerance and dependence to MT was found in mice. Further studies showed that MT could enhance the functions of T and B lymphocytes and macrophages in vitro and in adjuvant arthritis, and inhibit the disturbance of immune cells. MT could inhibit the swelling of hindpaw induced by carrageenin and complete Freund's adjuvant. These factors suggest that MT possesses marked antiinflammatory, immunoregulatory, and analgesic effects which may be related to the system of opiate, monoamine, and Ca2' modulation. Drug Dev. Res. 39