Administration of the antioxidant vitamin E to rats, prior to administration of either streptozotocin or alloxan, provided protection against the diabetogenic effect of both these agents. This was demonstrated by their response to a glucose load, their pancreatic insulin content and light microscopy findings. In addition, rats whose antioxidant state was depleted, by being maintained on a vitamin E and selenium-deficient diet, demonstrated increased diabetogenic susceptibility to normally nondiabetogenic doses of streptozotocin. These findings provide indirect support for the suggestion that the chemical agents streptozotocin and alloxan may exert their diabetogenic effect by acting as oxidants or free radical producers.