Perfusion studies of the terminal ileum were performed in patients who had previously had vagotomy and pyloroplasty performed for peptic ulcer disease and who at follow-up study had either diarrhoea or dumping, or no symptoms (the control group). The aim was to elucidate possible pathophysiological factors contributing to the sequelae. Net movements of water, electrolytes, and sugars, unidirectional fluxes of sodium and chloride, and the transmural electrical potential difference (PD) were measured simultaneously with and without the influence of chenodeoxycholic acid (CDC), 1.0 mmol/l, in the intestinal lumen. In patients without symptoms or with dumping, water and sodium chloride were absorbed during the control perfusion. Addition of CDC reduced the absorption or evoked secretion. In patients with diarrhoea the control perfusion disclosed a large spontaneous ileal secretion that was not influenced by CDC. The secretion was mainly due to a reduced mucosa-to-serosa flux of sodium and chloride and, to a lesser extent, to an increased serosa-to-mucosa flux. The diarrhoea patients showed an increased xylose absorption rate, suggesting an increased interepithelial leakiness. No change in PD was demonstrated. The effect of CDC on the unidirectional fluxes was identical in all three groups of patients. In conclusion, our findings suggest that the basic biological mechanisms are identical (and normal) in all patients, but the ileal epithelium of patients with diarrhoea secretes spontaneously and seems to be prestimulated by some at present unidentified agent.