Abstract: Recent studies have indicated that oxidative damage is involved in the toxicity of microcystins in animals.In the present study,the induction of oxidative stress by Microcystis cell extract in the liver of mice after prolonged exposure to cell extracts of Microcystis aeruginosa was investigated.The extracts were administered by intraperitoneal injection to Kunming mice at a daily dose of 3.3 μg microcystins/kg for 28 days.Toxin-induced damage of the liver was found by histological observation.Determination of lipid peroxidation demonstrated a significant increase in malondiadehyde in the toxin-treated mice compared to the control group.The activities of antioxidant enzymes(superoxide dismutase,catalase,and glutathione peroxidase) and the detoxification enzyme glutathione S-transferase in the liver of microcystin-treated mice were significantly increased compared to the control.These results showed that prolonged exposure to Microcystis cell extract led to oxidative stress in mouse liver,and oxidative stress might play an important role in the pathogenesis of microcystin-induced toxicity