Modifications of flow and composition of bile have been studied in the guinea pig with ligated ureters after injection of hypertonic solutions of two nonmetabolizable substances, mannitol and xylose (1.66 m), into the jugular vein. A distinct and persistent decrease in biliary flow always occurs. Analysis of this anticholeresis showed a parallel decrease in the flow of water and electrolytes, while the flow of bilirubin and bile acids is not modified. There are, then, two different types of mechanisms in the formation of bile: the osmotic mechanisms which control the quantity of water and electrolytes; and the active secretory mechanisms, independent of the first, for specific substances such as bilirubin and bile salts. Ultrafiltration does not occur in quantitative formation of bile, but the bile/blood osmotic gradient plays an important role.