The effect of sphincteroplasty on bile concentration and composition and on gallbladder function was investigated in the dog. Gallbladder and hepatic bile samples were analyzed for cholesterol, phospholipid (lecithin), bile salt concentration and individual bile salt content. Motor function was studied by cholecystokinin-cholecystography with changes in gallbladder volume computed from the radiographs. All bile samples were cultured and at the conclusion of the experiments, the gallbladders were histologically examined. Sphincteroplasty did not alter biliary cholesterol concentration but the concentration of lecithin and bile salts decreased in gallbladder bile and increased in hepatic bile (p less than .001). These changes depict a trend toward greater lithogenicity for gallbladder bile and lesser lithogenicity for hepatic bile. Postoperative analysis of individual bile salts in gallbladder bile showed an increase in monohydroxy and dihydroxy bile salts and a decrease in trihydroxy bile salts (p less than .001). This tendency has been shown to be conducive to gallstone formation. The concentrating ability of the gallbladder was partially eliminated by sphincteroplasty but gallbladder filling and motor response to stimulation by cholecystokinin was not affected. All gallbladders demonstrated histologic changes of chronic inflammation and all developed a significant bacterial flora following sphincteroplasty. It is concluded that cholecystectomy should always be performed following transduodenal sphincteroplasty not because of any resultant abnormality of motor function, as has previously been held, but because of the resultant abnormality of gallbladder pathophysiology.