To investigate the mechanism by which ablation of the sphincter of Oddi prevents gallstone formation, we assessed passage of glass beads out of the gallbladders of dogs with sphincterotomy and sham sphincterotomy. One month after bead implantation, dogs with an intact sphincter passed 52%, 26%, 22%, 10%, 0%, and 0% of beads with diameters of 2, 3, 4, 5, 6, and 8 mm, respectively. For the same respective bead diameters, dogs with a sphincterotomy passed 90%, 90%, 88%, 75%, 75%, and 42% of beads (p < 0.05 for all bead diameters). No beads were in the common bile duct of any dog. In separate dogs studied by cholescintigraphy, sphincterotomy significantly increased gallbladder ejection fraction from 0.46 to 0.76 (p < 0.01). In addition, sphincterotomy significantly lowered resting gallbladder volume from 24.4 to 15.8 ml (p < 0.025) and lowered cholecystokinin-stimulated gallbladder volume from 13.3 to 5.9 ml (p < 0.025). These data indicate that even with an intact sphincter, small solids can pass from the gallbladder and into the duodenum. Sphincterotomy facilitates passage of solids, apparently by general improvement in gallbladder emptying. Facilitated passage of crystals, microliths, or small stones seems the most likely explanation for prevention of gallstone formation by sphincterotomy.