A B ST RA CT Weanling rats were fed diets which contained either no protein or 27% protein. In one experiment after 23-35 days both groups were given l-leucine-4,5-3H either intragastrically or intraperitoneally and then sacrificed 24 hr later. In a second experiment animals were given these diets for 21 days and sacrificed 3, 6, or 12 hr after either intragastric or intraperitoneal administration of the labeled leucine. In both experiments the intestinal mucosa of proximal and distal segments of the small intestine was scraped, weighed, the protein concentration measured, and the specific activty of the mucosal protein was determined.The wet weight of the mucosa and the protein concentration of the mucosa were significantly greater in the control animals than in the protein-depleted animals. The mucosal protein per 100 g of body weight was the same in the protein-deprived and the control groups. The specific activity of the intestinal mucosal protein was higer in the protein-deprived animals than in the control animals.In the protein-deprived animals the proximal segment incorporated more radioactive amino acid into mucosal protein than did the distal segment at 3, 6, 12, and 24 hr after the amino acid was given by mouth. A similar difference was found between the proximal and distal segments of the control animals 6 hr after oral adminisstration of l-leucine-'H. On the other hand, when the l-leucine-3H was given intraperitoneally to both groups

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