12 pigs were divided into 4 groups. All animals received an identical basal diet deficient in lysine and additional isonitrogenous amounts of 66.4 mmol N in the form of 15N-lysine and 15N-urea resp. orally or caecally. Caecal application was carried out as permanent infusion through caecal cannulae. N- and 15N-balances were ascertained of all animals and the remain of the labelled nitrogen was determined. From the comparison of the N-balances the conclusion can be drawn that though caecally applied N-compounds, whether they were infused as amino acids or as non-amino acid-N, disappear in the large intestine, i.e. are digested, do not, however, improve the N-balance but are excreted as additional urine-N. Subsequent to oral application, lysine or urea are almost quantitatively absorbed in the small intestine. Absorbed lysine is used in the synthesis of body protein, absorbed urea, however, is almost completely excreted in urine (83% of the 15N-amount absorbed). 15N-excretion in faeces after the oral application of 15N-lysine and 15N-urea resp. was less than 1% of the 15N-amount applied, after caecal infusion, however, it was approximately 6%, the biggest part of which (70-77%) was incorporated in bacteria protein. After caecal infusion the main quota of the infused 15N-amount (greater than or equal to 80%) was excreted in urine, most of it in the form of urea. After the oral application of 15N-lysine this could be detected in both the TCA-soluble fraction of the serum and the serum protein. After caecal infusion 15N in the TCA-soluble fraction of the serum could mainly be found as NPN, absorption and incorporation of intact 15N-lysine were considerably lower. An calculation showed that the maximum of the absorbed 15N-amount in the form of lysine was 3% and that of the infused amount was 1.8%. It can generally be doubted that the absorption of lysine in the large intestine is significant in the protein metabolism. The absorption of utilizable lysine is practically completed at the end of the ileum. In the large intestine mainly the ammonia by the catabolic activity of the intestinal flora is absorbed and subsequently excreted through the intestines.