1. The extent to which ammonia-N (NH3-N) serves as a starting point for synthesis of microbial nitrogenous compounds was assessed when 15N as (15NH4)2SO4 was continuously infused into the rumen of a sheep for periods of 78–98 h. Steady states were reached in the composition of the rumen contents because the animal was fed equal parts of its ration at hourly intervals. Concentrations of 15N in bacterial-N, protozoal-N and rumen NH3-N were compared.

2. In two trials with a low-N diet consisting largely of wheaten hay the 15N concentration in bacterial N was 76 and 78% of that in the NH3-N. For protozoa the values were more variable —64 and 43%.

In two trials with a higher-N diet (lucerne hay), the corresponding values were lower—bacterial-N 62 and 64%, protozoal-N 41 and 35%.It was concluded that synthesis of microbial protein was more dependent on ammonia as a starting point with the low-N diet than with the higher-N diet.

3. Entry and exit rates for ammonia into and out of the rumen system were determined, and the results, in combination with those obtained for bacterial-N in the first part of the work, allowed calculations to be made of the production of microbial-N/d formed from NH3-N, and this in turn allowed calculation of minimal values for conversion of plant-N to microbial-N in the lumen. Minimal extent of conversion was 68% for the low-N diet and 53–55% for the higher-N diet.

4. Total production of microbial-N in relation to the amount of N given was also calculated by using previously reported values for the relative proportions of protozoal-N and bacterial-N in sheep given diets similar to those used here. These values for extent of conversion were 73% for the low-N diet and 58–59% for the higher-N diet.