In dogs with gastric fistulas and vagally denervated Heidenhain pouches, omeprazole, a benzimidazole derivative infused intravenously or given intraduodenally, dose-dependently inhibited gastric acid secretion, which had been induced by histamine, pentagastrin, or urecholine. It also suppressed gastric acid response to physiologic stimulants such as sham-feeding and gastric peptone meal without affecting serum gastrin level. The inhibition of histamine-induced acid secretion was accompanied by a parallel reduction in the mucosal blood flow, but no significant alteration in the ratio (R) value, indicating that omeprazole primarily affected gastric acid secretion but did not limit gastric mucosal microcirculation. Omeprazole, infused into the Heidenhain pouch, caused a dose-dependent inhibition of the Heidenhain pouch response to intravenous histamine without any significant change in the acid response of the main stomach and plasma concentrations of the drug. This indicates that omeprazole may exhibit local inhibitory action on the oxyntic glands. Omeprazole did not affect gastric mucosal integrity or the rate of alkaline secretion from the gastroduodenal mucosa or the pancreas stimulated by duodenal acidification or secretin.