This study was undertaken to evaluate (1) the colonic response to eating for a prolonged time in healthy subjects and patients with the irritable bowel syndrome (IBS); (2) the effect of octylonium bromide, a new smooth muscle relaxant acting by interfering with calcium ion mobilization, on the postprandial colonic motility; and (3) whether chronic gastric stasis could be responsible for both the dyspeptic symptoms often complained of by IBS patients and the faulty colonic response to eating. The colonic response to a 1000-kcal mixed meal in ten healthy subjects was characterized by two transient (from 0 to 60 and from 120 to 150 min postprandially, respectively) increases in colonic motor activity; ten IBS patients showed a continuous postprandial increase in colonic motor activity that was not terminated 180 min after eating. Treatment of IBS patients with octylonium bromide (80 mg, qid, per os) for 5-7 days reduced their colonic response to eating to a very short increase in colonic motor activity limited to the first 30 min. Finally, gastric emptying was not different in the two groups.