Four children with a mild non-insulin-requiring diabetes were studied. They had no insulin response at intravenous glucose tolerance test. When insulin was infused at a rate which simulated a normal early insulin response to intravenous glucose, blood glucose decreased to the same extent as it did in healthy subjects. When a normal early insulin response was simulated during the intravenous glucose tolerance test, the glucose assimilation rate was normalized. These results suggests that a peripheral resistance to insulin is unlikely in mild juvenile diabetes, and that the primary defect is a deficient release of insulin.