The activities of enzymes involved in cellular defence mechanisms such as superoxide dismutase, catalase, glutathione peroxidase and glutathione level have been found to be altered in experimental diabetes. Rats were made diabetic by a single i.v. injection of streptozotocin (55 mg/kg body weight) in citrate buffer. After the onset of diabetes, the diabetic rats were treated with sodium orthovanadate (0.3 mg/ml) for 15 days. Decreased activities of glutathione peroxidase, catalase, superoxide dismutase and glutathione content found in diabetic rats were corrected to near normal. The altered levels of plasma lipid peroxide, glycoproteins and erythrocyte membrane phospholipids in diabetic rats were restored to control levels by vanadate treatment. These observations clearly indicate the antioxidant potential of vanadate on experimental diabetes.