Thirty-eight non-obese and seven obese subjects were classified by a 2-hour glucose infusion test as having a normal carbohydrate tolerance. Sixteen non-obese ad ten obese patients showed a pathological carbohydrate tolerance (asymptomatic diabetes). For the characterization of insulin sensitivity a 1-hour priming dose-constant infusion technique consisting of two 30-minute-periods was applied in all subjects. A decrease of glycemia during this insulin infusion of less than 22.5 per cent has been accepted as a criterion of insulin insensitivity. Sixty-three per cent of non-obese and seventy-eight per cent of obese asymptomatic diabetics exhibited a reduced responsiveness to insulin. There ws an inverse relationship between the per cent decrease of plasma glucose concentration and the insulin secretion pattern revealed by the glucose infusion test. The results suggest that insulin resistance is a characteristic feature of obese and non-obese asymptomatic diabetics.