An extracorporeal "closed-loop" system has been employed to maintain glycemia in the normal range during consumption of meals in nine insulin-treated diabetics. This artificial pancreas system incorporated continuous blood glucose monitoring (0.05 ml. per minute, delay time 90 seconds), a computer programed to respond to glycemia, and a hormone delivery system. Intravenous insulin delivery rates were determined by control parameters responsive to both glucose concentration and its rate of change. Because insulin-dependent diabetics often defend themselves poorly against hypoglycemia (in some cases due to inadequate glucagon responses), the instrument was also programed for exogenous glucagon delivery. A priori selection of ideal parameters for insulin and glucagon delivery for each individual is not yet possible. Consequently, when the parameters were used for the first time on each subject, they were varied over a reasonable range. This approach resulted in a corresponding variety of glycemic responses, the average of which characterized a set of initial parameters that is generally applicable. Appropriate control parameters are presented that successfully prevented hypoglycemia. Glucagon delivery directly related to glycemia appeared sufficient for this purpose, thus obviating the need for dextrose administration. This system provides a technique for complete normalization of blood glucose concentration in the types of diabetics tested, during both fed and interprandial periods. It has yielded insights essential to the development of more sophisticated future devices.