1. Absorption of each of sixteen amino acids, free and peptide-bound, has been measured in isolated rat small intestine perfused with five partial digests of proteins. 2. At low concentrations net absorption of each amino acid was proportional to its luminal concentration and independent of the nature of the amino acid. 3. A series of first-order multiple regressions was found to describe well the characteristics of absorption. 4. Rate constants for disappearance of free and peptide-bound amino acids from the lumen were closely similar. However, substantial back-flux occurred of amino acids derived from peptide hydrolysis. Hence 60-70% of the amino-N entering the serosal tissue fluid probably had left the lumen as free amino acids. 5. Intact peptides crossed the mucosa during absorption from a soy bean hydrolysate and in substantial quantities during absorption from one casein digest but not from another. With other hydrolysates there was no evidence for passage of peptides to the serosa. 6. In several cases there was a serious discrepancy between the amount of amino-N absorbed from the lumen and the amount accounted for as peptide or free amino acid in the serosal secretion. 7. The characteristics of absorption were similar (apart from the exceptions in 5 above) for all the digests studied except for soy bean hydrolysate.