Postprandial gastrointestinal hormone secretion was not affected by the intravenous administration of naloxone. However, after the administration of morphine, multiple effects were observed. Postprandial pancreatic polypeptide, motilin, and enteroglucagon secretion were abolished and there was a reduction in the secretion of insulin, gastric inhibitory polypeptide, and neurotensin. There was prolonged secretion of gastrin. Levels of vasoactive intestinal polypeptide, somatostatin, and pancreatic glucagon were not affected. Many of these changes can be accounted for by the ability of morphine to delay gastric emptying. The prolonged secretion of gastrin could result from a combination of impaired gastric emptying and reduced postprandial gastric acid secretion. Inhibition of vagal acetylcholine release is the most likely explanation for the abolition of pancreatic polypeptide secretion. With the exception of the effects on motilin, pancreatic polypeptide, and enteroglucagon secretion this study provides no evidence that opiate substances directly affect the secretion of gastrointestinal hormones.