Normal subjects and diabetic patients with various degrees of hyperglycemia were given continuous intravenous glucose infusions for three hours in order to achieve constant plasma levels of glucose and immunoreactive insulin. During the last ninety minutes of this infusion period, at which time steady state conditions were observed, blood was obtained for measurement of plasma glucose and immunoreactive insulin concentration. Absolute levels of plasma insulin produced by the stimulus of the glucose infusions were extremely variable from patient to patient and were relatively independent of the degree of hyperglycemia. Patients with diabetes did not have lower steady state plasma insulin concentrations as a group, and this was true whether the insulin response was considered in absolute or relative terms. It is concluded that significant differences do exist in individual sensitivity to endogenous insulin, and that hyperglycemia in patients with maturity onset diabetes is not simply a function of lack of insulin.