Metformin is a biguanide derivate used as an oral hypoglycaemic drug in diabetics. The aim of this study was to examine the histological and biochemical effects of metformin in streptozotocin (STZ)-treated rats. The animals were rendered diabetic by intraperitoneal injection of 65 mg/kg STZ. Fourteen days later, metformin was given at 25 mg/kg by gavage, daily for 28 days, to STZ-diabetic rats and a control group. In the STZ-diabetic group, some degenerative changes were observed by light microscopic examination. But the degenerative changes were decreased in the STZ-diabetic group given metformin. In the STZ-diabetic group, blood glucose levels, serum alanine and aspartate transaminase (ALT and AST) activities, total lipid levels, and sodium and potassium levels increased, while body weight, serum magnesium levels and liver glutathione (GSH) levels decreased. In the STZ-diabetic group given metformin, blood glucose levels, serum ALT and AST activities, total lipid, and sodium and potassium levels decreased, and liver GSH and serum magnesium levels increased. As a result of all the morphological and biochemical findings obtained, it was concluded that metformin has a protective effect against the hepatotoxicity produced by STZ diabetes. © 2005 Edward Arnold (Publishers) Ltd.