Classical models of protein metabolism view nitrogen intake in tenns of the flux of free amino acids from dietary protein and their exchange between plasma and intracellular compartments and between freeand protein-bound amino acids (138). However, this view is misleading because it neglects any flux of amino acids through intennediate pools of small peptides, the least obvious of which is that produced by intracellular protein degradation (26). Other peptide pools comprise intennediates of dietary protein digestion, fil tered peptides in the renal tubular lumen, and circulating peptides from dietary sources or from intracellular protein degradation. These peptides enter the general free amino acid pool after peptidase hy drolysis, the total capacity of which must be as great as the whole-body flux of amino acids from protein turnover, currently estimated to be 1.5-4.5 g kg-1 d-1 (100-300 g d-1 for a 70 kg person) in healthy humans. Surgical stress, cancer cachexia, and liver disease can increase this capacity (35). Thus, whole body peptidase activity is capable of upregulation. Earlier studies defined the major organs involved in peptide metabolism (and in analogous glucose polymer metabolism)(Table 1). If all activity were expressed, the total capacity to hydrolyze dipeptides and maltose would be of the order of 4.4 kg d-1 and 230 g d-1, respectively. This hypothetical calculation explains why metabolism of iv synthetic dipeptides is so rapid compared with that of maltose.