Extracts from Petasites hybridus are in therapeutic use for more than 2000 years. They have been claimed to improve gastrointestinal pain, lung-diseases such as asthma and cough, as well as spasms of the urogenital-tract. We have investigated these claims in animal models of gastro-intestinal ulcers and confirmed that alcoholic extracts of Petasites hybridus block the ethanol-induced gastric damage and reduce small intestinal ulcerations induced in rats by indomethacin. Searching for a possible mode of action, we found that ethanolic extracts of Petasites hybridus inhibit the peptido-leukotriene biosynthesis in mouse peritoneal macrophages but leave prostaglandin biosynthesis unimpaired. We conclude that the active principle could be useful in the treatment of gastro-intestinal human diseases and that Petasites hybridus may contribute to the inhibition of calcium-dependent processes such as leukotriene biosynthesis.