The kinetics of secretion of gastrin and secretin before and after pancreaticoduodenectomy were studied. Prior to pancreaticoduodenectomy, gastrin secretory function was within the normal range; it was significantly reduced postoperatively. Preoperatively, patients had elevated basal levels of secretin with depressed reactivity. During pancreaticoduodenectomy, the upper part of the jejunum should be preserved as far as possible. Ulcers occurring after pancreaticoduodenectomy are more probably due to vagus nerve activity and other factors rather than to an imbalance between gastrin and secretin.