The authors investigated the nitrogen content of different fractions and in particular the aminoacid composition of the trichloroacetic acid (TCA)-precipitable and TCA-soluble peptides in the distal small intestine of rats which had been fed experimental diets (nitrogen-free or containing casein, wheat gluten or molasses yeast as a protein carrier). Parallel with this, they determined in growing rats the nitrogen and amino-acid digestibility of molasses yeast and wheat gluten according to the technique of faecal analysis, and studied the amino-acid composition of the faecal protein. Independently of the dietary protein fed, the peptide fractions of the content of the intestine showed an amino-acid composition that was largely similar to that found in case of nitrogen-free diet. The amino-acid composition of the faecal protein is, independently of the dietary protein given, relatively constant and corresponds approximately to the intestinal loss protein. There is agreement between the amino-acid composition of the TCA-precipitable peptide fractions and those of the corresponding faecal proteins. These components seem to be of endogenous origin.