In unanesthetized cats, continuous intravenous infusion of aspirin for 36 hr did not produce gastric ulcers when given alone but did when combined with 160 microgram kg-1 hr-1 of histamine-2HCl intravenously. The ulcers were mainly antral in location. The incidence and severity of ulcers increased with duration of the infusion up to 36 hr and with dose of aspirin up to 4 mg kg-1 hr-1. With the highest doses and longest durations some of the antral ulcers perforated. Antral ulcers occurred in more than half of the cats getting 0.25 mg kg-1 hr-1 or more of aspirin for 36 hr or getting 4 mg kg-1 hr-1 of aspirin for 6 or more hr. Intravenous aspirin plus intragastric infusion of 40 ml hr-1 of 150 mM HCl for 16 hr also produced gastric ulcers. Plasma salicylate concentrations were less than 350 microgram ml-1 with all doses and durations of aspirin used (400 microgram ml-1 is regarded as the upper limit of the therapeutic range in man). These studies show that when the stomach is acidified by giving histamine intravenously or HCl intragastrically, intravenous aspirin produces large deep gastric ulcers. The mechanism of the ulcerogenic action of intravenous aspirin is not known.