Effects of KW-5805, a new antiulcer agent, on various experimental ulcers, necrotizing agent-induced gastric lesions and gastric acid secretion in rats were compared with those of pirenzepine and cimetidine. KW-5805 showed antiulcer activities against experimental gastric and duodenal ulcers (ED50 = 1.2-10.0 mg/kg, p.o.). KW-5805 effectively inhibited gastric lesions induced by various necrotizing agents (ED50 = 4.5-39.8 mg/kg, p.o.). In addition, the cytoprotective effect of KW-5805 was not affected by indomethacin, but reserved by N-ethylmaleimide. These antiulcer and cytoprotective effects of KW-5805 were more potent than those of pirenzepine and cimetidine. In pylorus-ligated rats, intraduodenal KW-5805 administration at 30 mg/kg showed a weak antisecretory effect, which was 3-10 times less potent than those of pirenzepine and cimetidine. In rats with acute gastric fistula, intravenous injection of KW-5805 reduced methacholine-stimulated gastric acid secretion at doses of 10 and 30 mg/kg and inhibited tetragastrin-induced acid secretion at 30 mg/kg. These results indicate that KW-5805 has potent and broad antiulcer properties, which are probably exerted by its potent cytoprotective effect in addition to its antisecretory effect.