To gain a clearer understanding of the nature and composition of endogenous nitrogen containing substances lost from the upper mammalian digestive tract, digesta were collected from the terminal ileum of six growing pigs that had been fed a casein-based diet with titanium dioxide as an indigestible marker. Total nitrogen lost at the terminal ileum was in excess of 63 mg.g(-1) digesta dry matter. Of this, nearly 73% was proteinaceous, with nearly 45% being bacterial protein, 13% from soluble free protein, and 11% from mucin. Of the nonprotein nitrogen, 11% was as ammonia and 5% as urea. Bacterial and porcine cellular DNA nitrogen were collectively 0.2% of the total nitrogen. Only 8.3% of the total nitrogen remained unidentified and was assumed to include free amino acids, RNAs, amines, and the tetrapyrroles bilirubin and biliverdin. Although mucin contributed just 10.4% of the nitrogen losses, it was the single most abundant truly endogenous component, comprising 13% of the total dry matter. Bacterial nitrogen, combined with ammonia and urea nitrogen, represented nearly 61% of the total nitrogenous losses: this suggests substantial microbial activity in the stomach and small intestine of the pig. Centrifugal separation of a bacterial fraction from the digesta produced a microbial amino acid profile that, when subtracted from the overall amino acid content, provided an amino acid profile more representative of true endogenous amino acid losses.