Bile acids returning to the liver after reabsorption from the gut are removed from the plasma by an efficient, carrier-mediated process. They are conjugated (if necessary) and, together with newly synthesized bile acids, transported into the bile canaliculi. The latter process is saturable, is probably active, and its capacity far exceeds normal demands. Active bile acid secretion provides osmotic drive for canalicular water output and leads to excretion of other solutes by solvent drag and diffusion. There is also evidence for bile acid-independent canalicular secretion probably driven by active sodium transport. Choleretic humoral agents such as secretin, gastrin, cholecystokinin, and histamine may act upon sites in the ductular or ductal systems. Bile acid secretion provides the major drive for biliary lecithin and cholesterol excretion.