Pafenolol is a beta-blocker with unusual oral absorption properties. The blood concentration-time profile exhibits two peaks, and the bioavailability is low and dose dependent because of incomplete and nonlinear intestinal uptake. We addressed the question whether the intestinal absorption of pafenolol was affected by bile depletion in the gut lumen of rats. Further, the hypothesis that variable gastric emptying accounts for double peaks in blood was tested by duodenal administration of pafenolol. Following intraduodenal administration to rats with intact bile secretion, double peaks were observed in the blood concentration-time curve. The bioavailability was 6.8 +/- 0.7% for the low dose (1 mumol/kg) and increased significantly to 28 +/- 10% following the high duodenal dose (25 mumol/kg). These blood concentration-time profiles exclude interrupted gastric emptying as cause of the twin peaks. In bile duct-cannulated rats the intestinal absorption of the low dose (1 mumol/kg) was still poor (F = 10.7 +/- 5.5%) and the blood concentration-time profile contained two peaks. Following administration of a high duodenal dose (25 mumol/kg) to rats with an almost bile-free small intestine, the absorption rate increased and the double-peak phenomenon disappeared in five of seven rats, while the bioavailability increased significantly, to 62 +/- 27%. These results suggest that the low bioavailability of pafenolol is due to a complexation between bile and pafenolol in the gut lumen, preventing intestinal uptake in the major part of the small intestine. Further, such complex formation in the intestinal lumen may be the underlying mechanism of the double peaks observed in the blood concentration-time profile.