The mechanism(s) by which chronic glybenclamide therapy lowers the plasma glucose and improves diabetic control was studied in patients with adult-onset diabetes mellitus. Two months of therapy with glybenclamide resulted in a striking improvement in oral glucose tolerance associated with an increase in both plasma insulin and the insulin/glucose ratio. This improvement in glucose tolerance persisted through six months of therapy. However, the plasma insulin response to glucose and the insulin/glucose ratio decreased significantly toward pretreatment levels. During the study period there were no alterations in growth hormone secretion, thyroid function, adrenal cortical function or adrenal medullary function. The data suggest that glybenclamide, a new sulfonylurea drug, may improve diabetic control and improve glucose tolerance through a mechanism other than increased insulin secretion.