Pure cultures of rumen microorganisms usually grow better when amino acids are present, particularly if the amino acids are present as peptides. However, it is less clear how important peptides, compared to free amino acids and ammonia, are to the mixed microbial population of the rumen. In vitro mixed cultures suggest that peptides permit more rapid microbial growth on soluble carbohydrates and greater amino acid incorporation than free amino acids (Wright, 1967; Argyle & Baldwin, 1989). However, amino acid N may have no beneficial effect if the energy source is more slowly degraded (Merry et al., 1990). The present experiment was undertaken to assess whether the response of rumen fermentation to additional nitrogen is different depending on whether the N is present as non-protein N, free amino acids, or peptides in sheep receiving a grass hay diet.