To determine the effects of physiological and pharmacological insulin concentrations on leucine-carbon kinetics in vivo, eight postabsorptive normal volunteers were infused with L-[4,5-3H]leucine and alpha-[1-14C]ketoisocaproate (KIC). Insulin concentrations were sequentially raised from 8 +/- 1 to 43 +/- 6 and 101 +/- 14 and to 1,487 +/- 190 microU/ml, while maintaining euglycemia with adequate glucose infusions. At the end of each 140-min insulin-infusion period, steady-state estimates of leucine and KIC rates of appearance (Ra), KIC (approximately leucine-carbon) oxidation, nonoxidized leucine-carbon flux [an index of leucine incorporation into protein (Leu----P)], and leucine and KIC interconversion rates were obtained. After the three insulin infusions, leucine Ra decreased by a maximum of approximately 20%. KIC Ra decreased by a maximum of approximately 50%. The sum of leucine plus KIC Ra in the basal state was 2.59 +/- 0.24 mumol X kg-1 X min-1 and decreased by approximately 30% at the maximal insulin concentrations. KIC oxidation decreased by a maximum of approximately 65%. Leu----P did not increase after hyperinsulinemia. Interconversion rates were promptly and markedly suppressed by 50-70%. Leucine clearance increased by approximately 120%. We conclude that euglycemic hyperinsulinemia, at physiological and pharmacological concentrations, decreased leucine and KIC concentrations, leucine-carbon turnover and oxidation, and leucine and KIC interconversions in a dose-dependent manner in vivo.