With the use of a 75 g oral glucose tolerance test, both insulin release (IRG) and the degree of peripheral sensitivity (SI) were evaluated simultaneously in groups with normal (NGT) and impaired (IGT) glucose tolerance as well as NIDDM. IRG was expressed as the ratio of the area under the insulin curve to that of the glucose curve above fasting levels. The peripheral glucose uptake rate (M) during the OGTT was measured as the difference between the glucose load and the increase in the amount of glucose in the glucose space during the oral glucose tolerance test (OGTT). SI was expressed as the ratio of the metabolic clearance rate (M/mean blood glucose) to log mean serum insulin. In the non-obese groups, both mean IRG and mean SI values were decreased with an increasing degree of hyperglycemia from NGT to NIDDM. Decreased mean SI values were also found in obese subjects. IGT-subjects given 3 months of diet and exercise achieved improved SI values. A non-obese NIDDM-group had higher mean IRG and mean SI values after 6 months of treatment with glipizide. The results were comparable to data obtained with more complicated techniques, such as the insulin clamp and suppression tests, and should be easy to apply on a large scale in epidemiological studies.