The interdigestive gastroduodenal motor activity was studied in 7 patients with active duodenal ulcer and increased gastric acid secretion, in 7 patients with hypersecretory gastroduodenitis and in 7 subjects with normal acid secretion, in whom the increase in acid secretion was obtained by means of the intravenous administration of an H2 agonist, Impromidine. The gastroduodenal motor activity was recorded manometrically for about 200-300 min in basal conditions to obtain at least two subsequent activity fronts of the migrating motor complex (MMC cycle). Only in subjects with normal acid secretion was Impromidine administered at a dose of 2 micrograms/kg/h for 150 min followed by a dose of 10 micrograms/kg/h for another period of 150 min, to obtain, respectively, a submaximal and a maximal secretory response. Patients with spontaneous acid hypersecretion, with or without peptic ulcer, showed a longer than normal MMC cycle with a shorter than normal percent of time occupied by phase III. This motor activity was similar to that recorded in normal subjects during the increase in acid secretion induced by the lowest dose of Impromidine, whereas during the highest dose the gastroduodenal MMC was disrupted and replaced by an irregular motor activity.