The increase in tension of the gastroesophageal high pressure zone after fundoplication and hiatal hernia repair could represent either a mechanical effect or enhanced smooth muscle tone of the lower esophageal sphincter. Determination of the response of this high pressure zone to cholinergic stimulation, a protein meal and deglutition before and after Belsey Mark IV fundoplasty in seven patients with intractable reflux indicates the latter. Fundoplication resulted in a resting tension of the high pressure zone significantly greater than that found preoperatively but similar to that of controls. This normotensive postoperative high pressure zone had properties of a physiologic lower esophageal sphincter, that is, normal relaxation to deglutition, a response to a protein meal and a response to cholinergic stimulation with edrophonium. In response to edrophonium, evidence of a partial improvement in lower esophageal sphincter smooth muscle function was found postoperatively when compared with the response of controls and in patients preoperatively. The response to a protein meal postoperatively remained similar to that noted preoperatively, which was significantly less than the response of controls. Therefore, the increase in tension of the high pressure zone after fundoplication and hiatal hernia repair appears to result from improved function of the lower esophageal sphincteric smooth muscle.