Net glucose and free amino nitrogen absorption and insulin output in the pig hepatic portal vein were calculated from portal-arterial concentration differences and portal blood flow measured by thermodilution. Duodenal and jejunal motility were assessed by electromyography. In the fed pig, the migrating myoelectric complex (MMC) pattern of motility persisted; glucose and amino nitrogen absorption and insulin secretion all varied with the phase of the MMC, rising to maximum values 19.4 +/- 3.1 min before and falling to minimum values 16.7 +/- 4.4 min after phase III. The variations in glucose absorption with the MMC were caused primarily by changes in duodenal and jejunal motility rather than by changes in the rate of gastric emptying as glucose absorption and insulin secretion varied with the phase of the MMC during duodenal glucose infusions, with maximum values occurring 24.4 +/- 3.3 min before and minimum values 14.9 +/- 2.5 min after phase III activity. The mean glucose absorption time calculated by timing the appearance of glucose in the portal vein following an intraduodenal bolus injection was significantly longer when given in phase I showing that absorption was less rapid when the intestine was not contracting. A significant amount of glucose is metabolized by intestine, as during glucose infusion (500 mg min-1) the average portal glucose absorption (376 mg min-1) was less than the glucose infused.