The aim of this study was to test in vivo (a) whether the sphincter of Oddi acts as a resistor or also as a pump, (b) the effect of an IV infusion of cholecystokinin (CCK) on choledochoduodenal flow, and (c) the ability of the choledochoduodenal junction to prevent duodenobiliary reflux in two animal species, opossums (n = 11) and cats (n = 8). Opossums were implanted with bipolar electrodes on the sphincter of Oddi and the adjacent duodenum. Cats were not. Experiments were performed in vivo using a propulsion evaluation system to test whether the Sphincter of Oddi was able to pump fluid from the bile duct to the duodenum against pressure gradients. In 5 opossums and 4 cats, choledochoduodenal flow was evaluated during the IV infusion of CCK (20 ng.kg-1.min-1). The opossum sphincter of Oddi moved fluid against duodenal pressure gradients of 6-45 cm H2O. The spike-burst frequency (6.4 +/- 1.7 min-1) was maximal at peak bile duct pressures and decreased as bile duct pressure decreased (4.9 +/- 1.6 min-1; P less than 0.001). Pressure pulses in the bile duct were observed at a frequency that correlated with sphincter of Oddi spike-burst frequency (r = 0.84; P less than 0.001). In cats, choledochoduodenal flow occurred only along a hydrostatic gradient; the sphincter of Oddi never acted as a pump but only as a resistor. Infusion of CCk significantly increased the frequency of sphincter of Oddi contractions in opossums, but the transfer of fluid from bile duct to duodenum was significantly reduced. In cats, the rate of fluid flow from the bile duct to the duodenum during CCK infusion did not differ from control values. Reflux of duodenal fluid into the biliary tree was never observed, even at duodenal pressures as high as 100 cm H2O. In conclusion, in vivo, the sphincter of Oddi is able to pump fluid from the bile duct to the duodenum against a pressure gradient in opossums, but, in cats, choledochoduodenal flow requires a bilioduodenal pressure gradient. The increase in sphincter of Oddi contraction frequency induced by CCK in opossums resulted in a decrease in active transsphincteric flow. Duodenobiliary reflux could not be elicited in opossums and cats under the conditions of these experiments.