A Brahman x Hereford and a Hereford steer fed on a chaffed tropical pasture hay were infused intravenously and intraruminally with urea at 9–45 g N/day. By comparing the respective increases in rumen ammonia concentration which occurred when urea was infused intravenously and intraruminally, it was possible to estimate the net flow of urea into the rumen resulting from the intravenous administration. In both steers there was a limit to the net amount of urea passing from the blood to the rumen of c. 17–20 g N/day, which was reached at plasma urea concentrations of c. 12 mg N/100 ml. This concentration of plasma urea was achieved in the Brahman x Hereford steer when c. 23 g N/day was infused intravenously but in the Hereford when 32–35 g N/day was infused.