The levels of free, peptide-bound and protein-bound amino acid nitrogen in endogenous N at the distal ileum of the pig were determined when the digestion of protein after sample collection was minimized. Six 14-kg pigs were fed a protein-free diet and samples of digesta from the distal ileum were collected. The digesta were centrifuged and the supernatant ultrafiltered. On average, 86% of the total N in the digesta was found in the precipitate + retentate (greater than 10,000 mol. wt.) fraction, and in this fraction 78% of the total N was protein N. Of the total N in the ultrafiltrate (less than 10,000 mol. wt) the majority (69%) was peptide N. Amino N (free amino acid plus peptide) in the ultrafiltrate was 11% of the total N in the whole ileal digesta. Non-amino N (urea N + ammonium N + creatinine N) was only 0.73% of the total N in the whole ileal digesta. The mean amounts of nitrogen from individual amino acids (free + peptide) in the ultrafiltrate (expressed as percentages of the total nitrogen for the respective amino acid in whole ileal digesta) were low, ranging from 3.0% for isoleucine to 16.6% for proline. The finding that free amino acid N + peptide N is a small fraction of total N in endogenous ileal digesta supports the use of a new method for determining endogenous loss, whereby the animal is fed hydrolyzed casein and the ileal digesta are ultrafiltered. The high-molecular-weight fraction provides a measure of endogenous nitrogen loss.