1.15NH4Cl was continuously infused for periods of 120–216 h into the rumens of sheep which were allowed to feed 2 out of every 10min. These treatments achieved steady metabolic states and allowed the assessment of nitrogen conversions by means of tracer methodology. The sheep were given either a barley diet or one of three hay diets. In two trials, the flow of abomasal material was determined using lignin and polyethylene glycol as markers. The amounts of dry matter (DM), gross energy, total N, soluble N, microbial N and microbial DM in abomasal digesta, and the concentration of ammonia in the rumen liquor were measured. The concentrations of15N in the N of urine, faeces, rumen and abomasal bacteria and protozoa, rumen and abomasal bacterial and protozoal protein, abomasal particulate matter and in rumen ammonia were determined.2. Comparisons of the steady-state concentrations of15Nin the microbes with that in rumen ammonia indicated that from 50 to 65% of the bacterial N and from 31 to 55% of the protozoal N were derived from rumen ammonia in vivo.3. An amount of N equivalent to 60–92% of the daily intake was transformed into ammonia N in the runen.4. Some 17–54% of the ammonia was absorbed from the rumen, but this was not readily converted into urea.5. Microbial growth in the rumen resulted in the assimilation of 1.7–2.6 g N/100 g DM fermented.6. The generation-time of bacterial protein in the rumen was calculated from the rate of in- crease of15N concentration in this fraction, and values of 38 and 42 h were obtained for sheep given barley and hay diets respectively.7. The combined results allowed quantitative estimates to be made of the complete metabolism of rumen N, and from these the possibility of fixation of N gas in the rumen was suggested.