Inhibition of gastric acid secretion by the combined use of cimetidine and pirenzepin has been studied in 5 cases of Zollinger-Ellison syndrome. In 4 acute cases i.v. cimetidine and i.m. pirenzepin were administered together as a treatment and were successful in 3 out of the 4 cases as judged by the disappearance of clinical symptoms and healing of the gastroduodenal and jejunal mucosal ulcerations. Secretory measurements indicated that addition of pirenzepin to cimetidine resulted in larger and more prolonged inhibition of acid secretion in these patients. The continuous record of intragastric pH in the 5th patient, over 24-hour periods, confirmed the above findings. Despite the use of large doses of pirenzepin, no significant side effects were noticed.