Alimentary and cardiac autonomic nervous function was assessed in 25 patients with the irritable bowel syndrome. The vagally mediated increase in lower oesophageal sphincter pressure induced by abdominal compression was below that of 25 controls in 13 patients. Efferent vagal function, assessed by the ratio of peak acid output after insulin-induced hypoglycaemia to maximal acid output after pentagastrin, was subnormal in 7 of 23 patients. Pulse rate variability with deep respiration was subnormal in 6 of 23 patients. Abnormality in these tests did not correlate closely with the presence of oesophagitis at endoscopy or with that of gastro-oesophageal reflux on pH monitoring. Thus abnormalities in autonomic nervous reflexes might account for the frequent occurrence of gastro-oesophageal reflux and may be involved in the production of disordered gastrointestinal motility in irritable bowel syndrome.