1. Four young Friesian bulls with rumen fistulas were given four isocaloric all-concentrate diets containing different amounts and sources of nitrogen in a Latin square arrangement. Diet HP (high-protein) contained 2·31% plant nitrogen; diet MPU (medium-protein with urea) 1·67% plant nitrogen and 0·69% urea nitrogen (total 2·36%); diet LPU (low-protein with urea) 0·95% plant nitrogen and 0·69% urea nitrogen (total 1·65%); diet HPU (high-protein with urea) 2·28% plant nitrogen and 0·69% urea nitrogen (total 2·97%), calculated on an air-dry basis.
2. The rumen pH varied between 5·8 and 6·1 with diets HP, MPU and HPU, but was significantly lower with diet LPU with values between 5·4 and 5·8.
3. The results showed no differences between the isonitrogenous diets HP and MPU except that replacement of plant nitrogen with urea was followed by an increase in the concentration of ammonia in the rumen. With the diets containing urea, the concentrations of rumen ammonia varied inversely with the amount of dietary plant nitrogen supplied, indicating a negative effect of plant nitrogen on urea utilization.
4. Concentrations of alkali-labile nitrogen (amide) were not increased with diets containing urea except with diet HPU, which produced the highest concentrations of ammonia in the rumen.
5. The concentration of true protein in the rumen and the amino acid distribution were similar with all four diets, indicating the ability of the microflora to adapt to qualitative and quantitative differences in dietary nitrogen intake.
6. Ration acceptability was lower with diets LPU and HPU than with diets HP and MPU.
7. Large differences between individual animals in rumen pH, percentage of dry matter and total nitrogen concentration in the rumen were noted.