Chronic prophylactic exogenous insulin treatment commenced in young diabetes susceptible BB rats has been shown to prevent type I diabetes. This study was undertaken to examine whether this diabetes protection resulted from inhibition of beta-cell insulin secretion by exogenous insulin administration or from either a metabolic (chronic hypoglycemia) or immune effect of this treatment. We compared the effects of prophylactic exogenous insulin treatment with those of an insulin secretion inhibitor, diazoxide, an oral hypoglycemic agent, glyburide, and, water alone as controls in randomly divided BB diabetes-prone littermates treated from age 30 to 150 days. These experiments confirmed that exogenous insulin can prevent type I diabetes in the BB rat. Diazoxide, which inhibits endogenous insulin secretion while causing hyperglycemia (rather than hypoglycemia with insulin), also offered protection from diabetes. In contrast, the oral hypoglycemic agent glyburide, which increased insulin secretion, but decreased plasma glucose during the early part of the experiment, did not affect the incidence of diabetes. The lymphocyte subpopulations were unaffected by these treatments. These data support the hypothesis that decreased beta-cell activity is responsible for the protection against the immune beta-cell destruction.