Normal human volunteers were intubated with either aspiration tubes or a biopsy capsule placed in the small intestine. The subjects were then fed a test meal containing 50 g of purified bovine serum albumin which served as the model dietary protein. Electrophoretic analysis of intestinal fluids showed that for at least 4 h the fed albumin was detectable in jejunal and ileal fluids. On separate occasions, subjects were fed the same meal without the protein. No protein was detected in intestinal fluids when the protein-free meal was fed. After the protein-rich meal, total concentrations of measured free and peptide amino acids rose from 3.21 to 29.29, and 15.94 to 117.97 mumol/ml, respectively, (P values < 0.02) in the jejunum. Similarly, total concentrations of measured free and peptide amino acids rose from 5.45 to 19.74, and 13.59 to 65.39, respectively, (P values < 0.05) in the ileum. In contrast, concentrations of free and peptide amino acids in intestinal fluids did not increase after the protein-free meal. While intracellular concentrations of amino acids in the jejunal mucosa did not show significant changes, plasma concentrations of each individual free amino acid were increased after the protein-rich meal and were either decreased or unaltered after the protein-free meal. The amino acid composition of the fed protein was reflected in the increases in intraluminal and plasma concentrations of individual amino acids after the protein-rich meal. It is concluded that after the ingestion of a test meal containing a substantial amount of protein which is within the usual range of dietary intake; (a) the exogenous protein is the principal source of the increased free and peptide amino acids in the intraluminal contents and in the plasma; (b) there are greater amounts of amino acids present as small peptides than in the free form in the gut lumen; (c) the ingested protein can be recovered as late as 4 h both in the jejunum and in the ileum.