The aim of the study was to elucidate the differential role of the jejunum and ileum in the regulation of secretion of the gut hormones, gastrin, gastric inhibitory polypeptide, and enteroglucagon, and the pancreatic hormones, insulin, glucagon, and pancreatic polypeptide, in man. We measured the plasma levels of the hormones (and glucose) during fasting and after a test meal in 34 obese patients, of whom 5 were waiting for bypass surgery and 29 had had a jejunoileal bypass with a 3:1 or 1:3 jejunoileal ratio between the functioning segments 3, 9, or 15 months earlier. The major findings were that surgery bypass (1) has no important influence on the levels of gastrin and pancreatic polypeptide, (2) reduces the level of gastric inhibitory polypeptide, insulin (and glucose), and enhances the pancreatic glucagon level, independently of the jejunoileal ratio, and (3) increases enteroglucagon secretion, most effectively so with a short jejunal and long ileal segment left in continuity. These findings suggest that the upper jejunum and terminal ileum has no important role in regulation of secretion of these hormones apart from that in secretion in enteroglucagon which is related to the length of functioning ileum.