1. To establish the role of endogenous urea as a source of faecal ammonia, the plasma urea of two healthy men was labelled with 15N at a constant level for several days and its 15N enrichment was compared with that of faecal ammonia and total nitrogen.
2. Faeces collected after one complete gastrointestinal transit from the onset of plasma labelling had ammonia 15N enrichments which were only 8.5 ± 1.2% and total nitrogen enrichments which were 6.8 ± 0.7% of the plasma urea 15N enrichment.
3. These results show that endogenous urea is not the main precursor of faecal ammonia, which is probably derived by bacterial deamination from the protein of dietary residues, intestinal secretions and shed epithelial cells. The minor contribution of endogenous urea to faecal ammonia suggests that the lumen of the large bowel is not the main site of endogenous urea hydrolysis. The similar labelling of faecal total nitrogen and ammonia nitrogen supports other evidence that these faecal nitrogen fractions are in a constant state of exchange.