The choice of models relating tritiated water absorption to subepithelial blood flow in the rumen of sheep.

1. The blood flow to the ruminoreticulum of anaesthetized sheep was varied by changing the proportion of CO2 in the gas stirring solutions confined to this organ. 2. Clearance of tritiated water was thereby varied fourfold, from 10 to 44ml./min.100 g epithelium, with negligible net water movement. 4. Subepithelial blood flow, observed with radioactive microspheres using the reference organ technique, varied tenfold, from 20 to 200 ml./min.100 g epithelium. 4. The relation between tritiated water absorption and blood water flow was approximated by a hyperbolic model, but was appreciably better described using a higher order term in blood flow consistent with a countercurrent exchange of tritiated water between arterioles and venules. 5. The distribution of blood flow between different regions of the organ and between epithelium and muscle is described.