In order to investigate the relationship between dietary amino acids and protein, and activities of intestinal sucrase [EC 3.2.1.26] and leucine aminopeptidase [EC 3.4.11.1, LAPase] in rats, the effect of supplementation of amino acids into a protein-free diet and a low casein diet containing sucrose as the carbohydrate source on these enzyme activities was studied. The segmental weights of the small intestine and its mucosa of rats fed the protein-free diet supplemented with L-methionine or with L-methionine and L-threonine at 0.1 or 0.2% levels were significantly higher than those of rats fed the protein-free diet or one supplemented with L-glutamic acid, but there was no difference in the segmental activities of the sucrase and LAPase among rats fed these diets. On the other hand, the supplementation of methionine or methionine plus threonine to the 5% or 10% casein diet produced remarkable increases in the segmental weights of the small intestine and its mucosa as well as in the segmental activities of the sucrase and LAPase. There was no difference between the segmental sucrase activity of rats fed the 10% casein diet supplemented with 0.2% methionine ad libitum and that of rats fed this diet under restricted feeding conditions, although the segmental LAPase activity was affected by the amount of food consumed.