1. Re-examination of data for the chemical analysis of peptide-bound amino acids in the mesenteric venous blood of anaesthetized guinea-pigs suggests that there are small, but significant, amounts of small peptides in the blood of fasted animals.

2. Furthermore, there is a significant increase in the peptide content of the mesenteric venous blood during intraduodenal infusion of a partial digest of casein.

3. The data are consistent with the view that some 10% of the amino nitrogen entering the mesenteric blood during absorption of a casein digest in vivo may be in the form of small peptides, although it is not possible to define confidence limits for this estimate.

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