The contribution of protozoa to the protein entering the duodenum of sheep.

1. Four sheep, each fitted with a rumen fistula and a re-entrant cannula at the proximal duodenum were fed a semi-purified diet containing urea as the only nitrogen source. The quantities of total protozoal amino acid-N (TPAN) present in the rumen and entering the duodenum were determined when the mean rumen dilution rate (D) was low (0.034/h) and when D was increased to 0.078/h by the intraruminal infusion of artificial saliva. 2. Increasing the dilution rate had no significant effect upon the proportions of TPAN present in the total microbial amino-acid-N (TMAN) of the rumen fluid and duodenal digesta. With both dilution rates the mean proportion of TPAN in the duodenal TMAN (0.24) was markedly less than the equivalent proportion (0.45) found in the rumen fluid. 3. The daily flow of TPAN, as measured at the duodenal cannula at both dilution rates were equivalent to only 41% of the flow of TPAN as predicted from measurements of rumen outflow, indicating that a substantial proportion of rumen protozoal protein was retained within the rumen.