Except for branched chain amino acids, the site of indispensable amino acid degradation is the liver. Location of amino acid degradation capacity in a single organ may play an important role in the reutilization of amino acids derived from protein turnover. The importance of preferential utilization of amino acids for protein synthesis on catabolism of amino acids is demonstrated in two ways. First, by minimal oxidation of an amino acid at dietary concentrations below that required for maximum gain followed by a near proportionate oxidation with increased dietary level, and second, by increased oxidation of an indispensable amino acid when another amino acid limits protein synthesis. A direct effect of protein synthesis on amino acid catabolism can be shown by a marked increase in amino acid catabolism when protein synthesis is inhibited.