Two active compounds that prevent serotonin-induced ulcerogenesis in rats were isolated from Chinese cinnamon (the stem bark of Cinnamomum cassia) and identified as 3-(2-hydroxyphenyl)-propanoic acid and its O-glucoside, The former compound, administered orally or parenterally to rats at a remarkably low dose (40 µg/kg body weight), also inhibited gastric ulcers induced by the other ulcerogens such as phenylbutazone, ethanol, and water immersion stress, although it failed to prevent in-domethacin-induced ulcers. Pharmacological studies have shown that 3-(2-hydroxyphenyl)-propanoic acid hardly inhibited the secretion of gastric acid, but promoted the gastric blood flow. These results suggest that the antiulcerogenic effect of this compound is probably attributable to the potentiation of defensive factors through the improvement of the circulatory disorder and gastric cytoprotection.