From the ripe dried fruit of the Chinese plant Rhododendron molle G. Don, we extracted and isolated a crystalline substance named “Rhomotoxin”. Rhomotoxin appears as a white crystal or crystalline powder, with a molecular weight of 368, its empirical formula being C20H32O6. It is the active hypotensive and bradycardiac principle of the fruit, and its biological potency is qualitatively the same as that of the original crude drug. In this paper the method of extraction and isolation of rhomotoxin, its chemical and physical properties, and its drug standard are presented