目的　探讨褪黑素对大鼠乙酸性结肠炎的影响及有关机制。方法　制备大鼠乙酸性结肠炎模型 ,实验设正常对照组、模型对照组、阳性药物对照组 (5 氨基水杨酸 ,10 0mg·kg-1)、MT给药组 (2 5 ,5 0 ,10 0mg·kg-1) ,采用灌肠方式给药 ,每天 1次 ,给药时间从制备模型 2 4h后开始至实验结束共 7d ,正常及模型对照组均给予生理盐水灌肠。实验结束后观察大鼠结肠粘膜损伤指数 (CMDI)、粪便隐血实验(OB)、髓过氧化物酶 (MPO)含量和粘膜病理组织学 (HS)情况 ,并检测结肠组织丙二醛 (MDA)、谷胱甘肽过氧化物酶(GSH Px)、过氧化氢酶 (CAT)、超氧化物歧化酶 (SOD)、一氧化氮 (NO)含量。结果　乙酸性结肠炎大鼠结肠CMDI、HS、OB程度和MPO水平均比正常组明显升高 ,MT灌肠可减轻乙酸性结肠炎大鼠的CMDI和粪便OB程度 ,降低MPO水平 ,10 0mg·kg-1MT可改善结肠粘膜病理损伤。同时可见模型组大鼠结肠组织MDA、NO含量增加 ,SOD、GSH Px和CAT水平降低。MT可明显降低MDA、NO含量 ,增加GSH Px、SOD和CAT水平。结论　MT对乙酸性结肠炎大鼠结肠粘膜损伤具有保护作用 Objective To investigate the effect and mechanism of melatonin on acetic acid-induced colitis in rats. Methods The rat model of acetic acid-induced colitis was established. The normal control group, the model control group, the positive drug control group (5-aminosalicylic acid, 100 mg · kg -1) and the MT-treated group (2.55, 10 0 mg · kg-1). The rats were administered with enema once daily. The administration time ranged from 24 hours after the preparation of model to 7 days after the end of experiment. The normal and model control groups were given normal saline enema. At the end of the experiment, the colonic mucosal injury index (CMDI), occult blood test (OB), myeloperoxidase (MPO) and mucosal histopathology were observed. The malondialdehyde (MDA) , Glutathione peroxidase (GSH Px), catalase (CAT), superoxide dismutase (SOD) and nitric oxide (NO) were measured. Results The levels of CMDI, HS, OB and MPO in colon of rats with acetic-induced colitis were significantly higher than those in normal rats. MT enema could reduce the levels of CMDI and stool OB in rats with acetic-induced colitis and reduce the levels of MPO, -1MT can improve the pathological damage of colonic mucosa. In the meantime, the content of MDA and NO in the colon tissue of the model group increased while the levels of Px and CAT of SOD, GSH decreased. MT can significantly reduce MDA, NO content, increase GSH Px, SOD and CAT levels. Conclusion MT can protect colonic mucosa of rats with acetic-induced colitis