Abstract: Melamine（MA） is a low-acute toxicity chemical with LD50 of 4 550 mg/（kg·bw） in mice.To verify and evaluate its chronic toxicity,500,1 000 and 2 000 mg MA per kilogram feed were fed to 6-week-old female Wistar rats for six months.The results showed that functional disorders and organic changes of kidney,liver and ladder of female Wistar rats to different extent were observed in each test group.The blood biochemical indices（such as ALT,AST,CRE and UA） increased to different extent.The histological abnormalities or damages to different extent were found in kidney,liver and bladder.In 2 000 mg MA/kg dose group,the growth and feed intake were inhibited and histological change or damage of kidney was significant,even kidney calculi appeared.In poisoned group,no histological abnormality was found in spleen and bone marrow of rats.Maximal no-effect dose of MA on rats should be less than（35.7±15.4） mg/（kg·bw·d）.