Coordination of gallbladder and sphincter of Oddi and effect of cholecystectomy on biliary pressure physiology were investigated in 7 patients using an indwelling microtransducer catheter placed in the bile duct by duodenoscopy. Intramuscular morphine (0.2 mg/kg) to induce a sphincter of Oddi spasm produced no change before cholecystectomy but intravenous caerulein (0.1 microgram/kg) induced pressure elevation coincident with gallbladder contraction on echogram. After cholecystectomy, however, morphine caused a pressure rise and a coarse and irregular change of the tracings, which seemed attributable to sphincter of Oddi spasm. Caerulein promptly reduced the pressure and eliminated the irregularity. These results show (a) the sphincter of Oddi relaxes when the gallbladder contracts in response to caerulein and (b) the gallbladder acts as a pressure reservoir against the sphincter of Oddi spasm caused by morphine. The latter implies that the spasm of the sphincter of Oddi readily leads to a pressure rise if the gallbladder is absent, which may partly explain a development of postcholecystectomy syndrome.