Abstract: Objective To explore the effects of exogenous methionine(Met) on distribution of a rsenicals and nitric oxide metabolism in the brain of mice exposed to arsenite in drinking water.Methods Forty mice were divided randomly into 5g roups with 8 mice in each group.The mice in the exper miental groups were exposed to drinking water contam inated with 50 mg/L arsenic for four consecutive weeks.At the fourth week,the mice treated intraper itoneally with Met at doses of 100,200,and 400 mg/kg·bwand saline solution separately.Twenty four hours afterce ssation of Metadm in istration,the mice were anaesthetized and dissected rapidly.Sam ples of blood and brain were collected mimediately for analysis of arsenic species.Levels of inorganic arsenic(Ais),monomethy larsenic acid(MMA) and dmiethy larsenic acid(DMA) were determ ined with HG AAS method.Activities of nitric oxide synthase(NOS) and the concentrations of nitricoxide(NO) were determined with kits purchased from Nanjing Jiancheng Co.Results Compared to those treated with saline solution,adm inistration of Met significantly decreased the levels of Ais,MMA and total arsenic levels(TAs) in the blood and the levels of Ais,DM A and TAs in the brain.A ctivities of NOS and the levels of NO in As exposure group were significantly lower than those of in the control.However administration of Metcouldame liorate these toxic effects and NO levels in the group treated with 400mg/kg·bw.Met were significantly higher than those in saline solution treated group.Conclusion Exogenous Met could promote methylation of a rsenic,there fore reduce arsenic levels in both blood and brain.Moreover,administration of exogenous methionine couldame liorate the effects of arsenic on NO metabo lism in the brain via decreasing arsenic burden.