Alkaloid in Oxytropis was extracted, separated and appraised, Swainsonine was found in the alkaloid extract. Rats were fed forage mixed with oxytropis or alkaloid, two group rats became poisoned and showed similar clinical symptoms, autopsy and histopathologic changes, which indicated that the alkaloid was the principal toxin. Not only conventional H. E. staining, but also histochemical technology and transmission electron microscopy were used to observe the histopathologic changes in the poisoned rats.