Among the powerful dipeptidases of the cytosol fraction the well-characterized 'master' dipeptidase of broad substrate specificity is an example of a 'true' dipeptidase. There are only a limited number of peptidases which together can hydrolyse the total range of the theoretically possible dipeptides. In the lumen, the dipeptides of dietary origin apparently enter the cell through a single transport system of broad specificity in the monkey and possibly also in man. Perfusion studies indicated impared absorption of glycine and Gly-Gly in Indian compared to English subjects, but the kinetic advantage of dipeptide uptake over that of the free amino acid was maintained. In patients with tropical sprue, apart from decreased peptide absorption, there was an increased 'backflow' of the constituent amino acids. The back-flow assumes nutritional significance since dipeptide uptake is competitively inhibited by amino acids of a certain specificity. Inhibition of brush border glycylglycine hydrolase (glycylglycine dipeptidase, EC 3.4.13.1) by L- leucine is non-competitive while that of cytosol enzyme is competitive. Certain other amino acids have no effect on glycylleucine hydrolase (glycylleucine dipeptidase, EC 3.4.13.2) from either fraction but inhibit Gly-Leu uptake. Thus, the inhibition of dipeptide uptake appears to be a consequence of interaction of amino acids directly with the dipeptide transport system and not with dipeptidases.