Test rats were labelled with 15N over a period of 7 days and killed on the 12th day. 220 min. before they were killed the diet was changed (wheat diet leads to whole egg diet) and they were given a 14C-leucine injection. The 15N-labelling of the digesta proved to be suitable for the determination of the digestibility of the feed proteins in the individual sections of the intestines. If the atom-% 15N-excess of the TCA-soluble fraction of the digesta is = 100%, the resulting digestibility of the protein fraction (TCA-precipitable fraction) corresponds to the 15N-dilution of the unlabelled feed protein. The following digestibility values were ascertained: (formula; see text) The calculation method suggested here cannot be applied to the large intestine because microbial activity influences the quotients. 14C-labelling was also insufficient for gathering statements on the digestibility of proteins.