OBJECTIVE: In this study, it was aimed to investigate the antioxidant effect of metformin treatment on liver and kidney tissues in mice with experimental type 2 diabetes induced by a high-fat diet and streptozotocin.MATERIAL AND METHODS: Twenty-one adult C57BL6 male mice were randomly divided into three groups as Control group (C), Diabetes group (D), and Diabetes + Metformin group (D + MTF). Groups D and D + MTF were fed a high-fat diet with 60% fat for four weeks. After the 4th week, 30 mg/kg streptozotocin was applied intraperitoneally, and type 2 diabetes was induced. 300 mg/kg/day Metformin was given by gavage. Glutathione (GSH), catalase (CAT) and malondialdehyde (MDA) levels in liver and kidney samples were measured by spectrophotometric method. Tissue sections were stained with Hematoxylin and Eosin and evaluated histomorphologically.RESULTS: The results showed that MDA levels increased in the liver and kidney tissues of diabetic animals. It was found that MTF treatment decreased MDA levels in both tissues and increased GSH and CAT levels, but this increase was not significant in the liver (p> 0.05), while it was significant in the kidney (p <0.05). In addition, in the histopathological evaluation results, vacuolization, occasional necrosis and synozoid occlusion were observed in rare areas in the D+MTF group, while intense vacuolization, high rates of synozoidal occlusion and necrosis were observed in group D livers. In kidney tissue, the tubular structure in the group C was normal while the tubules were enlarged in group D, and there were dense bleeding areas. In the D+MTF group, vacuoles were observed in the tubule structures in rare areas.CONCLUSIONS: The results of this study show that oxidative stress occurs in tissues in diabetes and that metformin treatment prevents tissue damage by showing a corrective effect on the increased oxidative stress in diabetes.