The interdigestive gastroduodenal motility was studied by means of a manometric probe in 6 patients with active duodenal ulcer and acid hypersecretion, in 6 patients with ulcer disease in remission (inactive) and normosecretion and in 8 healthy subjects with normosecretion. After a basal recording period sufficient to record at least two activity fronts of the migrating motor complex (MMC), an intraluminal infusion of isotonic NaHCO3 was carried out for 180 min in patients with active duodenal ulcer, whereas in patients with ulcer in remission an HCl solution was infused for 180 min. Patients with active duodenal ulcer showed a basal motility with a longer than normal MMC cycle and a shorter than normal activity front, while patients with ulcer in remission showed a cyclic motor activity not significantly different from that of normal subjects. The NaHCO3 infusion in patients with active ulcer restored a near-normal motility, whereas the HCl infusion in patients with ulcer in remission induced a motility similar to that of patients with active ulcer. These data indicate that the increase in gastric acid secretion is responsible for the decrease in frequency and duration of MMC activity fronts, which have the function of cyclically clearing the gastroduodenal lumen. Consequently, acid and bacteria may remain a longer than normal time in contact with the gastroduodenal mucosa, which, in this manner, may be greatly exposed to the risk of peptic lesions.