In 24 control patients the lower oesophageal sphincter responded to graded increments in intra-abdominal pressure by a significant and sustained rise in tone. This response was abolished by atropine and was also absent in nine of 11 patients who had previously undergone truncal vagotomy for duodenal ulcer but not in the remaining two who had recurrent ulceration. In six patients studied after proximal gastric vagotomy a variable response was seen. Gastric efferent vagal function was assessed by a combined insulin/pentagastrin gastric secretory test and did not correlate closely with the lower oesophageal sphincteric response to increased intra-gastric pressure. These findings are explicable if it is assumed that truncal vagotomy interrupts the afferent limb of a reflex arc regulating lower oesophageal sphincteric tone.