The antioxidant effects of Ocimum sanctum in experimental streptozocin-induced diabetic rats was evaluated in this study. Streptozocin, 55 mg kg(-1) body weight, was injected intraperitoneally once daily for 30 days to induce diabetes mellitus in rats. Streptozocin-induced diabetic rats were orally treated with an aqueous extract of O. sanctum once daily for 30 days. After the experimental period, thiobarbituric acid reacting substances (TBARS) and antioxidant enzymes such as catalase, superoxide dismutase (SOD) and glutathione peroxidase were measured. Administration of O. sanctum to streptozocin-induced diabetic rats for 30 days significantly reduced the plasma level of TBARS and improved the status of the antioxidant enzymes catalase, SOD and glutathione peroxidase in vital organs such as the liver and kidney. These results confirmed that the Indian medicinal plant O. sanctum has a protective effect and it may be useful in controlling complications resulting from diabetes.