The influence of different N sources on fermentation rate and de novo amino acid synthesis by rumen micro-organisms was investigated in vitro using rumen fluid taken from four sheep receiving a mixed diet comprising (g/kg DM): grass hay 500, barley 299·5, molasses 100, fish meal 91, minerals and vitamins 9·5. Pancreatic casein hydrolysate (P; comprising mainly peptides with some free amino acids; 10 g/l), free amino acids (AA; casein acid hydrolysate + added cysteine and tryptophan; 10 g/l), or a mixture of L-proline, glycine, L-valine and L-threonine (M; 0·83 g/l each) were added to diluted (1:3, v/v), strained rumen fluid along with 15NH4Cl (A; 1·33 g/l) and 6·7 g/l of a mixture of starch, cellobiose and xylose (1:1:1, by weight). P and AA, but not M, stimulated net gas production after 4 and 8 h incubation (P < 0·05) in comparison with A alone. P increased microbial-protein synthesis (P < 0·05) compared with the other treatments. All of the microbial-N formed after 10 h was synthesized de novo from 15NH3 in treatment A, and the addition of pre-formed amino acids decreased the proportion to 0·37, 0·55, and 0·86 for P, AA, and M respectively. De novo synthesis of amino acids (0·29, 0·42 and 0·69 respectively) was lower than cell-N. Enrichment of alanine, glutamate and aspartate was slightly higher than that of other amino acids, while enrichment in proline was much lower, such that 0·83–0·95 of all proline incorporated into particulate matter was derived from pre-formed proline. Glycine, methionine, lysine, valine and threonine tended to be less enriched than other amino acids. The form in which the amino acids were supplied, as P or AA, had little influence on the pattern of de novo synthesis. When the concentration of peptides was decreased, the proportion of microbial-N formed from NH3 increased, so that at an initial concentration of 1 g peptides/l, similar to the highest reported ruminal peptide concentrations, 0·68 of cell-N was formed from NH3. Decreasing the NH3 concentration at 1·0 g peptides/l caused proportionate decreases in the fraction of cell-N derived from NH3, from 0·81 at 0·53 g NH3-N/l to 0·40 at 0·19 g NH3-N/l. It was concluded that different individual amino acids are synthesized de novo to different extents by mixed rumen micro-organisms when pre-formed amino acids are present, and that the source of N used for synthesis of cell-N and amino acids depends on the respective concentrations of the different N sources available; however, supplementing only with amino acids whose synthesis is lowest when pre-formed amino acids are present does not stimulate fermentation or microbial growth.