The sphincter of Oddi (SO) regulates trans-sphincteric flow (TSF) by acting primarily as a pump or as a resistor in specific species. We used the Australian possum SO, that functions similarly to the human SO, to characterize SO motility responses to different common bile duct (CBD) and duodenal pressures. Possum CBD, SO and attached duodenum (n=18) was mounted in an organ bath. External reservoirs were used to impose CBD (0-17mmHg) and duodenal (0, 4, 7mmHg) pressure. Spontaneous SO activity was recorded using 4-lumen pico-manometry and TSF was measured gravimetrically. Temporal analysis of manometric and TSF recordings identified three functionally distinct biliary-SO regions, the proximal-SO (juxta-CBD), body-SO and papilla-SO. At CBD pressures <3mmHg the motor activity of these regions was coordinated to pump fluid. Proximal-SO contractions isolated fluid within the body-SO. Peristaltic contraction through the body-SO pumped this fluid through the papilla-SO (17-27µl contraction), which opened to facilitate flow. CBD pressure >3.5mmHg resulted in progressive changes in TSF to predominantly passive "resistor"-type flow, occurring during proximal-SO/body-SO quiescence, when CBD pressure exceeded the pressure at the papilla-SO. Progression from pump to resistor function commenced when CBD pressure was 2-4mmHg greater than duodenal pressure. These results imply that TSF is dependent on CBD/duodenal pressure difference. The papilla-SO is pivotal to TSF, relaxing during proximal-SO/body-SO pumping and closing during proximal-SO/body-SO quiescence. The pump function promotes TSF at low CBD pressure and prevents bile stasis. At higher CBD pressure, the papilla-SO permits TSF along a pressure gradient, thereby maintaining a low pressure within the biliary tract.