1. The rumen urea concentration in gnotobiotic lambs lacking ureolytic bacteria was equal to that of blood. 2. Bacterial urease (EC 188.8.131.52) activity in sheep fed by intraruminal and intra-abomasal infusion was inversely related to rumen ammonia concentration. 3. A model is proposed for the facilitation and control of urea flux by wall-found ureolytic bacteria.