Man controls his blood sugar concentration within rather narrow limits, despite wide latitude in the content and timing of meals. Man does not eat constantly; rather, he alternates between periods of "feasting" and "fasting." It is a useful generalization that the metabolic environment is altered such that forces promoting substrate storage are dominant during the postprandial period and forces that encourage substrate mobilization are prominent during food-free intervals. In this report, we shall primarily examine the role of insulin, the hormone of carbon storage, in maintaining glucose homeostasis during a 24-hr period. In particular, we wish to highlight (A) possible important differences in the dose response curves of insulin in splanchnic and peripheral tissues and (B) the modulating influences of hyperglycemia and glucagon on insulin's effects across the splanchnic bed.