Rumen fluid was removed from four sheep 6 h after feeding, and the fluid was centrifuged to remove the micro-organisms. Perchloric acid (PCA) was added to the supernatant fluid to precipitate soluble proteins, which were again removed by centrifugation. The PCA extract was neutralized with KOH and the precipitate was removed by centrifugation. The supernatant fluid was hydrolysed with 6 MHC1 at 110 °C for 24 h, then dried by rotary evaporation, and the amino acid composition of each extract was analysed by ion-exchange chromatography. The recovery of amino acids was 98%, except for methionine, cysteine and tryptophan, which were destroyed. The recovery of amino acids from added Trypticase was 92% of the peptide mixture as amino acids. The free amino acid composition of extracellular rumen fluid was low and variable in both amount and composition (0·36, S.D. 0·49, μmol/ml). The concentration of amino acids released by acid hydrolysis of the PCA extract, presumed to be derived from peptides, was larger and its composition was less variable (1 ·02, S.D. 0·30, μmol/ml). Aspartic acid and histidine were enriched in peptides in comparison with the amino acids present in the feed or in rumen particulate material. Glycine and proline contents were higher in peptides that in particulate material. In contrast, the contents of isoleucine, leucine, tyrosine and phenylalanine tended to be lower in peptides than in the other materials. It was concluded that extracellular degradation-resistant peptides had a composition that was different to microbial protein and to the feed. The peptides appeared to be enriched for amino acids which previous studies with pure peptides had shown tend to make peptides more resistant to degradation.