Increasing interest in the role of oxidative stress and beta-carotene in disease and prevention led us to examine the results of beta-carotene's administration in diabetic rats, a model for high-oxidative stress. In this experiment, amounts of lipid peroxidation, glutathione, and glutathione disulfide, and activity levels of catalase, glutathione peroxidase, glutathione reductase, superoxide dismutase, and gamma-glutamyl transpeptidase were measured in the liver, kidney, and heart of Sprague-Dawley rats with streptozotocin-induced diabetes, and after treatment with 10 mg/kg/day of beta-carotene for 14 days. Beta-carotene treatment resulted in the reversal of the diabetes-induced increase in hepatic and cardiac catalase activity, the decreased levels of glutathione disulfide in the heart, and the increased cardiac and renal levels of lipid peroxidation. Treatment with beta-carotene exacerbated the increased glutathione peroxidase activity in the heart and the decreased catalase activity in the kidneys. In contrast to reduced hepatic glutathione levels in untreated diabetic rats, beta-carotene treatment increased glutathione levels in diabetic rats. Increased hepatic gamma-glutamyl transpeptidase activity in diabetic rats was not reduced by treatment. Thus, beta-carotene therapy for 14 days prevented/reversed some, but not all, diabetes-induced changes in oxidative stress parameters. Copyright 2002 Wiley Periodicals, Inc. J Biochem Mol Toxicol 16:203-208, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/jbt.10038