The aim of this study was to determine the rate of circulating gastrointestinal hormones in pigs when pancreatic secretion was re-introduced or removed from the duodenal lumen. Permanent fistulas were fitted into the pancreatic duct and duodenum of 16 pigs, and catheters were introduced into the portal vein and a jugular vein as well. Plasma hormone content was determined at different times during two periods of 30 and 60 min each, when the juice was returned to the pigs; these two periods included a 120-min interval when the juice was not reintroduced into the intestinal lumen. When the pancreatic juice was not returned to the pigs, plasma secretin content rose significantly in the portal blood (+ 42.8% at min 15; +52.5% at min 30; +31% at min 60 and +27% at min 120) and in the peripheral blood (+36.4% at min 30; +18.9% at min 120) compared to the mean values recorded during intraduodenal re-introduction of the secretion. In parallel, the volume of juice secreted and protein output increased significantly but their concentration did not. When the pancreatic juice was removed the mean cholecystokinin level in the portal vein did not change significantly, except at min 30 when its value was 32% higher than the mean during the periods of restitution. Cholecystokinin in the peripheral blood was not affected by removing the pancreatic juice from the intestinal lumen. The mean plasma levels of gastrin, somatostatin, VIP and PP in the portal and peripheric blood were unaltered. These results suggest that in pigs secretin may have an important role in the regulation of pancreatic secretion by negative feedback.