Insulin secretion was studied before and after the control of hyperglycemia in fourteen maturity onset male non-obese diabetics. Optimum control of hyperglycemia was achieved by the addition of the sulfonylurea chlorpropamide to dietary treatment. One patient was a primary treatment failure, but nine out of thirteen had excellent control of hyperglycemia. A standardized oral glucose tolerance test (GTT) was performed before and after eight months of individualized therapy with the sulfonylurea. The GTT was repeated with each patient taking his usual dose of chlorpropamide 90 min prior to the administrationo f the glucose load. In the baseline test glucose levels rose from 135.6 +/- 9.9 mg/dl to a peak level of 268.8 +/- 17.7 mg/dl at 120 min. After control of hyperglycemia glucose levels were significantly lower at 0, 30 and 60 min, and rose from 106.8 +/- 8.5 mg/dl to a maximum of 224.5 +/- 17.3 mg/dl at 120 min. Plasma insulin response was unchanged. Fasting serum cholesterol, triglyceride and total lipid levels changed only minimally during therapy. It is concluded that lowered serum glucose levels after long term treatment with chlorpropamide occured while plasma insulin response to glucose was no greater than before treatment. These findings may be explained by an extrapancreatic effect of the drug or by an indirect result of chlorpropamide induced insulin release which occured earlier in the course of therapy.