The possibility that gastrointestinal hormones influence biliary lipid secretion was studied further in chronic bile fistula dogs and subsequently in a patient with a balloon-occludable t-tube. After stabilization of bile flow in the dog by infusing 500 mg/hr sodium taurocholate, glucagon and insulin increased bile flow, decreased cholesterol output, and had no effect on phospholipid or bile salt output. Similarly, during reinfusion of bile in the human subject, glucagon increased bile flow, decreased cholesterol output and had no effect on bile salt output. Phospholipid secretion decreased, as had occurred at a lower bile salt infusion rate in the chronic bile fistula dog. These findings suggest that glucagon or insulin may play a role in regulating cholesterol secretion in dog and man, and that mechanisms other than simple dilution of micelles are operative.