C14 glucose was adminisered continuously to unanesthetized normal dogs by intravenous infusion following a priming dose at a rate which maintained the specific activity of the circulating glucose at a nearly constant level. Glucagon-free insulin was then administered intravenously in varying doses. Samples of blood were collected at intervals throughout the experiment and more frequently just subsequent to insulin injection. The glucose concentration of the circulating blood and the specific activity of this glucose, when considered together, allow calculations to be made of a) the outflow of glucose toward the tissue cells, and b) the inflow of glucose from the liver during the period after insulin injection. By this means it was shown that increased glucose uptake by the tissues is much more important than decreased delivery of glucose by the liver in bringing about insulin-induced hypoglycemia. It was also shown that recovery from hypoglycemia is brought about by increased glucose delivery by the liver and not by a decrease in tissue uptake of glucose below the preinsulin level. Alternative methods of calculation of absolute values for glucose inflow and outflow during periods of changing blood glucose concentrations are discussed and evaluated.