"Alkaloid A" was isolated from the Chinese drug I-Mu-Tsao,Leonurus sibiricus L.by alumina column chromatography of the crude ethanolic egtraot of the plant.The acid-degradation produots of "alkaloid A" were proved to be syringic acid,4-gnanidino-l-butanol and nitrous acid.The complete structure of "alkaloid A" was identified as leonnrine nitrite by comparison with an authentio sample.