Ammonia plays an important role in providing N for protein synthesis by rumen microorganisms. Studies using [15N]ammonia or [15N]urea indicate that the proportion of microbial nitrogen derived from ammonia is highly variable, ranging from 18 to 100% (Nolan, 1975; Salter et al., 1979). Peptides and amino acids are also incorporated by rumen microorganisms. They generally stimulate growth (Argyle & Baldwin, 1989), but little is known about the factors which govern the relative proportions of microbial amino acids which are derived from pre-formed amino acids and from ammonia. The present study was undertaken to determine how the concentration of peptides affects ammonia incorporation and the de novo synthesis of individual amino acids by mixed rumen microorganisms.