The aim of this study was to examine lipid peroxidation and activities of key antioxidant enzymes in kidneys of rats with streptozotocin (STZ)‐induced diabetes and the effect of aminoguanidine on diabetes‐induced alterations. Three groups, 6 rats each, were studied: control animals, not treated diabetic rats and rats treated with aminoguanidine (AG; 1 g/liter of drinking water). After 6 and 12 weeks the animals were sacrificed and lipid peroxidation products and activities of antioxidant enzymes were determined in their kidney homogenates. Malondialdehyde (MDA) content was significantly elevated and activities of SOD and catalase decreased in the kidneys of STZ‐diabetic rats. AG treatment attenuated the increase in MDA content and diminutions of activities of SOD and catalase in the kidneys of diabetic rats. These results confirm oxidative stress in the kidney of rats with STZ diabetes and point to an antioxidant effect of AG in experimental diabetes.