The effect of dietary amino acids on jejunal sucrase (EC 3.2.1.26) and leucineaminopeptidase (EC 3.4.11.1, LAPase) activities in rats was studied. Rats were force-fed a 10% complete amino acid diet or valine-free diet. The sucrase and LAPase activities in rats force-fed the valine-free diet for 2 days were significantly lower than those in rats force-fed the complete amino acid diet, although the specific activities of these enzymes in the isolated brush border fragment were 10 times higher than those in the mucosa, and most of the activities of these enzymes in the mucosa were localized in the isolated brush border fragment. Results of experiments undertaken to investigate the effects of dietary amino acids during the initial period after the dietary alteration on the sucrase and LAPase activities showed that decreases in the activities of these enzymes in rats force-fed the valine-free diet appeared by 26 hours after the first feed administration; whereas, incorporation of dietary 14C-amino acids administered in the first feed administration into the mucosal protein was significantly lower in rats receiving the valine-free diet than in rats receiving the complete amino acid diet by 7 hours following the first feed administration. These results suggest that decreases in availability of dietary amino acids in the valine-free diet for protein formation in the small intestinal mucosa during the initial period caused the decreases in the sucrase and LAPase activities localized in the brush border membrane.