The study was conducted to investigate the effect of melatonin (MEL) on the expression of β-endorphin (β-EP) in the hypothalamic arcuate nucleus (ARH) of morphine-dependent mice. For a period of 8 consecutive days, male Kunming strain mice were injected subcutaneously (s.c.) with normal saline or increasing doses (10-80 mg/kg) of morphine, and intraperitoneally (i.p.) with MEL (10, 20 or 40 mg/kg) or vehicle (5% ethanol saline) simultaneously. Withdrawal response was induced by naloxone (3 mg/kg, s.c.) at 2 h after final morphine injection on the 8th day. The potency of withdrawal response was evaluated according to the jumping times and the body weight loss. After that, the expressions of β-EP and proopiomelanocortin (POMC) mRNA in ARH were examined by immunohistochemistry and RT-PCR, respectively. The results showed that MEL (i.p., 20 mg/kg) decreased the naloxone-precipitated withdrawal responses in morphine-dependent mice significantly (P<0.05). Meanwhile, MEL increased the intensity of β-EP-like immunoreactivity and enhanced the expression of POMC mRNA in ARH (P<0.05). These results suggest that MEL increases the expression of β-EP in ARH of morphine-dependent mice, which may partly contribute to the action of MEL to inhibit the development of morphine dependence.