The dose-response relationship between the hepatic sinusoidal insulin level and glucose production by the liver is such that a half-maximally effective concentration is at or slightly below the hormone levels seen basally after an overnight fast. In the normal individual, the direct effect of the hormone on the hepatocyte is far more important in restraining glucose production than its indirect effect mediated via a suppression of lipolysis. Because insulin regulates the liver in a direct fashion, its effect occurs within several minutes. Thus, the speed with which insulin works and the sensitivity of the liver to it predict that first-phase insulin release should have a significant effect in quickly suppressing hepatic glucose production. On the other hand, nonhepatic tissues are much less sensitive to insulin and respond slowly as a result of the need for insulin to cross the endothelial barrier. As a result, first-phase insulin is unlikely to significantly alter peripheral glucose disposal. Simulation studies in humans and dogs in which the effects of first-phase insulin were simulated confirmed the aforementioned predictions. In addition, they confirmed the ability of second-phase insulin release to have significant effects on both glucose production and utilization.