Glucose, insulin and somatostatin infusion over 2 hours effectively suppressed endogenous secretion of insulin, glucagon and growth hormones. Steady state plasma glucose level (SSPG) which should be inversely proportional to insulin sensitivity was obtained. In 6 adult-onset non-obese untreated diabetics, mean value of insulin sensitivity indices was significantly reduced compared with normal. In 5 insulin-treated diabetics and in 5 subjects with borderline glucose tolerance including 2 obese subjects, insulin sensitivity for glucose utilization was also significantly diminished.