Mixed bacteria were obtained from the rumen contents of steers and were taken at different times after the animals had been given different feeds. The feeds contained [15N]urea and in some experiments Na2 35SO4 so that the bacteria were labelled with these isotopes. 2. Samples of labelled bacteria were subjected to a simulated abomasal digestion with pepsin and the digests were infused with a non-absorbed marker, polyethylene glycol, into the duodenums of test steers equipped with re-entrant ileal cannulas. Except for samples taken after a 24 h fast, which sometimes gave somewhat lower values, mean values for digestibilities of 15N and 35S in the small intestine were 0-79 and 0-85 respectively. 3. The corresponding value for the digestibility of 15N in similarly-treated wheat-leaf protein concentrate was 0-86. 4. Net digestibilities of total N in the small intestine of the test steers when they were given either a mainly protein-free diet of straw, tapioca and urea or a diet of flaked maize and hay were approximately 0-62. 5. From these and other values it was calculated that approximately 5-2 g intestinally-secreted endogenous N/d flowed through the ileum of a 100 kg steer.
