Superoxide dismutase was administered intravenously to rats 50 min prior to intravenous administration of a diabetogenic dose of streptozotocin. A dose of 45 mg/kg streptozotocin alone produced marked glucose intolerance and a decrease in pancreatic insulin content to less than 10% of control; both of these effects were abolished by prior administration of 105 u/g of superoxide dismutase. Superoxide dismutase (105 u/g) administered 50 min before 65 mg/kg intravenous streptozotocin did not prevent the development of diabetes. The fall in pancreatic insulin content seen with streptozotocin alone was, however, partially reversed by superoxide dismutase.