Transport and metabolism of L-carnosine (beta-alanyl-L-histidine) were studied in rat small intestine. Carnosine administered orally was found in rat serum as well as small intestine and liver, followed by an increase of histidine. At ten minutes after carnosine infusion per os, the carnosine content of the hepatic portal vein increased with the dose. On the other hand, the histidine content increased two-fold but did not vary with the dose. These results suggest that part of the carnosine administered orally is hydrolyzed to beta-alanine and histidine in the small intestine. Carnosinase activity was present in many rat tissues and was most active in kidney in the presence of Mn2+. However, in the absence of Mn2+ carnosinase activity in small intestine was found to be the same level as that of kidney. A study has been made of the distribution of carnosinase along the small intestine of adult rat. The dipeptidase was distributed along the whole length of the small intestine with maximum hydrolytic activity in the jejunum, and was localized in the cytosol of the intestinal mucosa. Antiserum prepared against carnosinase purified from kidney inhibited the activity of small intestine as well as that of kidney.