1. When rumen fluid together with small quantities of (15NH4)2S04 [1-14C]sodium acetate and polyethylene glycol (PEG) was introduced into the emptied rumen of a sheep, no significan change occurred in the ratio 15N:14C during a period of 30–60 min. The ratio 15N:PEG, however, declined by 22–53%. when(15NH4)2SO4, [1-14C]sodium acetate and PEG were introduced into the normal rumen of a sheep fed on lucerne hay the ratio 15N:14C in the whole rumen contents showed no appreciable change in a period of 1 h, but the ratio 15N:PEG declined by 25–41%. It was therefore concluded that the rate constants for removal of ammonia nitrogen (NH2-N) and volatile fatty acid (VFA) from the rumen were very nearly equal.
2. In sheep fed at 1 h intervals on lucerne hay chaff or lucerne hay pellets VFA production, VFA and NH3-N concentration in the rumen fluid and the volume of fluid passed from the rumen to the omasum were determined. From these results and the information previously gained on the relative rates of removal of NH2-N and VFA from the rumen, the amounts of NH3-N produced in the rumen and absorbed there or at lower levels of the digestive tract were calculated to be equivalent to 23–27% of the dietary N in lucerne hay chaff and 17% of that in lucerne hay pellets; 59–66% of the NH3-N was absorbed from the rumen and the remainder passed to the omasum.