Objective To study the sub-chronic toxicity of lake water treated with effect microorganisms(EM).Methods One hundred and ten Wistar rats were randomly divided into five groups,11 males and 11 females in each.Three groups were treated orally with lake water depurated with EM,at the doses of 125,250,500 ml/kg respectively,for 90 consecutive days.The other two groups were treated with the untreated lake water(250 ml/kg) and tap water,respectively.The animal's behavior,body weight,blood and organ tissue indicators were examined.Results No poisoning signs were observed in all animals,no significant difference was found in body weight,blood routine test indexes,the serum test indexes between all treated groups and control groups,and no significant pathological change in the organ tissues of all treated groups and control groups was seen.Conclusion The lake water depurated with EM at doses of 125 to 500 ml/kg may not induce sub-chronic toxicity in rats.