Anti-ulcer properties of 2-(3,4-dimethoxyphenyl)-5-methylthiazolidine-4-one (KM-1146) were investigated on various experimental ulcers in comparison with cimetidine. While both KM-1146 and cimetidine at the same dose were comparative on the preventive effects of Shay- and stress-ulcers, KM-1146 was more effective against different necrotizing agents such as 0.2N NaOH and 99.5% ethanol at non-antisecretory dose. KM-1146 50 mg significantly accelerated the healing of acetic acid-induced ulcers while cimetidine 200 mg had no effect. KM-1146 increased rat gastric mucosal blood flow as measured by hydrogen gas clearance and prevented a decrease of gastric mucus gel thickness induced by water immersion restraint stress. These studies indicate that KM-1146 exerts cytoprotective action even in anti-secretory dose in addition to a significant anti-ulcer action which is comparable to cimetidine.