Bile secretion and blood flow in the portal vein and the hepatic artery were determined in cholecystectomized anesthetized dogs before and during continuous infusion of varying doses of secretin. Secretin increases bile volume without alteration of bile acid output. Hepatic arterial flow was not altered by any dose. However, high doses of secretin increased portal venous blood flow significantly. It is concluded that secretin action on liver blood flow mirrors mainly superior mesenteric arterial vasodilation and seems to be a rather pharmacologic than a physiologic response. Bile secretion is not influenced by changes of hepatic blood flow in the physiologic range.