The effect of epinephrine on leucine and phenylalanine kinetics was measured by using the stable isotope amino acid tracers L-[1-(13)C]leucine and L-[phenyl-2H5]-phenylalanine in the postabsorptive state and during the intravenous administration of a standard amino acid solution with respect to the amino acid load. Infusion of epinephrine (plasma concentrationapproximately 3600 pmol/L) decreased leucine and phenylalanine and increased ketoisocaproate plasma concentrations and increased the metabolic clearance rate of leucine and phenylalanine. Epinephrine neither influenced leucine or phenylalanine flux nor leucine oxidation or leucine net balance. Hyperaminoacidemia from amino acid infusion reduced endogenous leucine release and stimulated leucine oxidation and nonoxidative disposal of leucine, resulting in a dose-dependent increase in leucine net balance. Epinephrine did not influence any changes in amino acid kinetics during parenteral amino acid administration. Therefore, we conclude that epinephrine had no catabolic effects on amino acid metabolism and no negative effect on the utilization of a parenterally offered amino acid solution in healthy humans.