Oral treatment with the H2-antagonist ICI 162,846 or omeprazole for five days inhibited both basal and pentagastrin stimulated acid secretion by 50% or more in mice. Either treatment increased the luminal secretion of histamine in the basal (12-fold) and stimulated (9-fold) states. Mice treated with the H2-antagonist had a 27% reduction (p less than 0.05) in mural histamine in the acid-producing area of the stomach. Mice were treated so as to induce duodenal ulcers (abscopal model) and were then treated with the H2-antagonist ICI 162,846, omeprazole or vehicle, orally for one week. Fewer duodenal ulcers were found in animals receiving drug treatments than in the oral vehicle group. Both the H2-receptor antagonist and the proton pump blocker inhibit acid production; acid blockade by either drug is accompanied by a massive increase in secretion of histamine. This rise was associated with depletion of the gastric histamine store only with H2-receptor blocker. Both means of acid inhibition reduce the formation of ulcers in this model.