Data documenting the isolated effect of systemic hyperinsulinemia on whole body and skeletal muscle leucine carbon kinetics in humans are limited. Using steady-state [14C]leucine kinetics, 10 normal volunteers were studied in the baseline postabsorptive state and then under euglycemic, hyperinsulinemic (71 ± 5 μU/ml), and euleucinemic conditions. Systemic hyperinsulinemia resulted in a significant decrease in whole body and forearm leucine rate of appearance (R(a)) by 17 and 37%, respectively, (P < 0.0003, 0.03), without a significant change in the nonoxidized rate of disappearance for either (P = 0.23, 0.66). The baseline contribution of total body skeletal muscle (TBSM) leucine R(a) and rate of disappearance (R(d)) to whole body leucine R(a) and R(d) was 27 ± 6 and 24 ± 5%, respectively. During hyperinsulinemia TBSM R(a) decreased by 34%, whereas whole body R(a) decreased by 16%. We conclude that the primary effect of insulin in the whole body and skeletal muscle is to decrease leucine release from protein without a significant effect on leucine incorporation into protein. This antiproteolytic effect of insulin is more pronounced in skeletal muscle than in other tissues in the body.