The acid response to a combined sham feeding-pentagastrin test was studied in 27 patients with gastric ulcer (GU), 47 patients with pyloric or prepyloric ulcer (PU), 65 patients with duodenal ulcer (DU), and 29 healthy subjects (HS). In each group basal acid secretion (BAO) and peak acid output to sham feeding (PAOSh) and to pentagastrin (PAOPg) were significantly correlated. After correction for body weight these variables were equivalent in men and women. DU patients showed basal hypersecretion, not only due to a greater secretory capacity, since the BAO/PAOPg ratio was significantly higher than in non-DU groups. The response to vagal activation was also increased in DU patients. Thus the ratio PAOSh/PAOPg amounted to 0.52 in DU, compared with 0.34-0.36 in non-DU subjects (p less than 0.001). The relative acid responses were similar in healthy subjects and GU and PU patients. Thus DU patients seem to have a vagal hyperfunction that might be due to defective inhibitory mechanisms, for which there is supporting evidence, or might be caused by a possible vagal hyperactivity, which is a speculation.