Experiments were carried out with rat intestine to investigate the interaction of various amino acids, peptides and peptidases in intestinal absorption. A number of amino acids reduced the rate of hydrolysis of glycylglycine and glycyl-L-leucine and in both cases L-histidine was the most effective inhibitor. With L-leucyl-glycine the effects were quite different, and most amino acids tested caused some stimulation of hydrolysis, except L-histidine which caused inhibition. There was no evidence of competition between amino acids and dipeptides for an entry mechanism, but there was evidence for competition between different dipeptides. Cobalt caused stimulation of dipeptidases in homogenates but not in intact intestine, and this suggested that the dipeptidase is not accessible to cobalt in the intestinal lumen. The results are discussed in relation to the terminal stages of absorption and digestion of protein.