Abstract: AIMTo observe the effect of melatonin (MT) on morphine withdrawal syndromes and determine the content of NO in plasma and brain tissue in morphine dependent mice. METHODSA physical dependent model in mice was established by subcutaneous injection of morphine. MT (15 mg·kg^-1, qd×3) was given by intragastric infusion (ig) for three days. Withdrawal syndromes were induced by intraperitoneal injection of naloxon (5 mg·kg^-1). The intensity of withdrawal syndromes was evaluated according to the jumping latency, the jumping times and the body weight loss. The content of NO was detected with Griess method. RESULTSThe jumping latency of morphine withdrawal reaction was prolonged and the jumping times were reduced obviously by ig MT. The increased NO content in plasma and brain tissue in morphine dependent mice was reduced by ig MT. CONCLUSIONThe physical withdrawal syndromes and the content of NO in plasma and brain tissue in morphine dependent mice are inhibited by MT.