Basal and histalog-stimulated gastric acid secretion and serum gastrin levels before and after a standard protein meal were compared in eight children with active duodenal ulcer (DU), four with active gastric ulcer (GU), and in seven children with recurrent abdominal pain (RAP) of undetermined etiology. There was no discernible difference in the pattern of abdominal pain in DU, GU, and RAP. Basal acid output, peak and maximal acid output, whether expressed as milliequivalents per hour or as milliequivalents per kilogram per hour, were comparable in children with DU, GU, and RAP. In contrast, serum gastrin levels, 1 and 2 hours after standard protein meal, were significantly higher in the DU children than in the GU or RAP group. These studies have suggested that hypersecretion of gastric acid may not be associated with duodenal ulcer or gastric ulcer disease in children, and that increased gastrin secretion and possible reduced acid responsiveness coexist in children with duodenal ulcers.