To determine the independent effects of insulin and glucose on the rate of appearance of leucine in humans, five normal volunteers were studied using a pancreatic clamp with sequential insulin infusions of 1.8, 3.6 and 7.2 pmol kg/min-1. On each of three separate occasions, the plasma glucose level was maintained at 5.0, 9.4 or 14 mM. The rate of appearance of leucine was similar at each insulin infusion rate regardless of the ambient glucose concentration or the glucose infusion rate. These studies demonstrate that during insulin and glucose infusion, insulin, but neither the plasma glucose concentration nor the rate of glucose utilization, primarily regulates leucine flux (a reflection of endogenous whole body proteolysis) in normal humans.