Our objectives in this study were (a) to determine the role of antroduodenal resistance in the control of fasting duodenogastric bile reflux in the dog and (b) to elucidate the contribution of the pylorus both to resistance and to reflux. Thus, we measured simultaneously throughout the interdigestive motor cycle (a) antroduodenal pressure activity by manometry, (b) antroduodenal resistance by a pneumatic resistometer, and (c) bile acid concentrations in duodenal and gastric juices. Experiments were performed in 15 conscious dogs (9 with pylorus intact and 6 with extramucosal pyloric myotomy). We found that antroduodenal resistance was lowest during phase I, increased gradually during phase II, and peaked during phase III (linear trend, p less than 0.001). Duodenogastric bile reflux was low during phase I, peaked during late phase II, and decreased again during phase III (quadratic trend, p less than 0.05). Therefore, variations in net resistance and reflux were differently related to the phases of the interdigestive motor complex. Pyloric myotomy significantly decreased antroduodenal resistance (linear trend different from control, p less than 0.001), but had no significant effect on duodenogastric bile reflux. We conclude (a) that changes in net antroduodenal resistance do not regulate duodenogastric bile reflux and (b) that the pylorus is an important determinant of antroduodenal resistance, but has no major role in the control of fasting duodenogastric bile reflux.