An increased incidence of gallstones has been reported following truncal vagotomy and gastric drainage but never conclusively proven. In the Rhesus monkey, bile composition and flow is similar to man. A model of biliary drainage was established which permits continuous monitoring of bile kinetics. Following truncal vagotomy and pyloroplasty, a significant fall in bile acid concentration was observed, accompanied by a rise in cholesterol concentration. This resulted in an overall rise in cholesterol saturation. Similar changes in hepatic bile composition were, however, also seen following pyloroplasty alone, and suggests that the vagotomy itself may not be the important factor in producing bile changes.