Mild hyperglycemia was induced in normal rats by oral administration of both diazoxide and D-glucose. After 48 hours of such a treatment, the insulin and glucagon secretory responses of the perfused pancreas to alpha- and beta-D-glucose (3.3 mM) were examined in the presence of 10.0 mM L-leucine. The output of insulin, but not that of glucagon, and the perfusion pressure were higher in treated than control rats. The alpha-anomer of D-glucose was a more potent insulin secretagogue than beta-D-glucose in both control and treated rats. However, the alpha/beta ratio in insulin output was twice higher in control than treated rats. By analogy with other experimental models of diabetes, the attenuation in the anomeric difference of glucose-stimulated insulin output in the treated rats could reflect an altered secretory response to alpha- rather than beta-D-glucose. These findings suggest that hyperglycemia provokes, as a function of its severity and duration, first attenuation and then suppression, if not inversion, of the anomeric preference for alpha-D-glucose in insulin release. They are also compatible with the hypothesis that the anomeric malaise, associated with B-cell glucotoxicity, is caused by a progressive accumulation of glycogen in this cell.