To investigate the mechanism of adaptive response of intestinal aminopeptidase on the dietary protein level, we determined the aminopeptidase activiO' and mRNA level in the intestinal mucosa of rats with different dietary protein levels. In the first experiment, the specific activity of aminopeptidase in the homogenate of intestinal mucosa fi'om rats fed a 60% casein diet for 7 days was significantly higher than that from rats fed a 20% casein diet. A larger difference between the two groups was observed in the ileum than in the jejunum. In contrast to aminopeptidase, the specific" activity of sucrase in the ileum of rats fed a 60% casein diet was significantly lower than that of rats fed a 20% casein diet. The activities of sucrase in the jejunum and alkaline phosphatase in the jejunum and ileum did not change between the m'o groups. Northern blot analysis of intestinal RNA using 1.1 kb aminopeptidase N cDNA as a probe did not show a significant difference in the level of aminopeptidase N mRNA between two dietary groups. In the next experiment, the aminopeptidase activity and mRNA level in the ileal mucosa were determined in rats fed O, 10, 20, 40, and 60% casein diets for 7 days. While the .spec!fic activity of aminopeptidase increased in response to the dietar)_ β’ protein level, sucrase and alkaline phosphatase activities did not change. However, we did not observe any change in the level of ileal aminopeptidase N mRNA in all groups, and there was no significant correlation between the specific activit), and the mRNA abundance. These data indicate that the catalytic activity of intestinal aminopeptidase responds to the dietary protein level, but the level of mRNA is preserved even under the protein malnourished condition. The data also suggest that translational and~or posttranslational regulations .for the expression of intestinal aminopeptidase are necessary ./br the rapid response to changes in intraluminal nutrients.