1. Plasma amino acid kinetics were determined in hospitalized patients receiving one of three intravenous solutions: isotonic amino acids, isotonic sodium chloride, or total parenteral nutrition.
2. Whole body amino acid appearance, oxidation and incorporation into protein were estimated with two different isotopically labelled amino acids: l-[1-14C]leucine and l-[U-14C]tyrosine.
3. A positive correlation was obtained between whole body amino acid appearance, oxidation and incorporation into protein with the two isotopically labelled amino acids.
4. Derivation of whole body protein kinetics with l-[U-14C]tyrosine consistently gave higher values than those obtained from l-[1-14C]leucine, presumably due in part to the contribution of phenylalanine hydroxylation to plasma tyrosine appearance. However, the percentages of amino acid appearance oxidized and used for protein synthesis were similar.
5. It can be concluded that estimates of whole body protein kinetics are qualitatively similar when measured with l-[U-14C]tyrosine and l-[1-14C]leucine in hospitalized patients receiving either isotonic sodium chloride solutions or balanced amino acid intakes.