Antral ulcers were produced in unanesthetized rats in 3 hr by simultaneous intravenous administration of acetylsalicylic acid (ASA) and gastric perfusion with 0.15 M HCl. Intravenous infusion of ASA alone or gastric perfusion with HCl alone produced no antral ulcers. Plasma salicylate levels ranged from 250 to 350 microgram ml-1. 16-16 Dimethyl prostaglandin E2 (DMPGE2) (0.04, 0.40, 4.0 microgram kg-1 hr-1) and cimetidine (10, 50 mg kg-1 hr-1) significantly decreased the severity of antral ulcers in a dose-dependent fashion. In a separate group of unanesthetized rats prepared with gastric fistula and pylorus ligation, pepsin output during administration of parenteral ASA and gastric perfusion of HCl was reduced by the highest doses of 16-16 DMPGE2 and cimetidine. However, addition of exogenous pepsin to the HCl perfusate had no effect on the inhibition of ulceration afforded by 16-16 DMPGE2 and cimetidine. We conclude that both 16-16 DMPGE2 and cimetidine protect antral mucosa against injury by parenteral ASA plus topical HCl by some means other than their effect on acid and pepsin output.