By a series of experiments, we identified a new member of the locoweed family, Oxytropis serioopetala, that produces swainsonine, a phytotoxin harmful to livestock. In order to evaluate the toxicity of Oxytropis serioopetala, its extract was administered to ten rabbits by gavage at a dose of 1.5 mg/kg body weight as swainsonine once daily. After the 20th day, the rabbits appeared depressive and anorexic. In addition, intention tremors were apparent upon movement. Their eyes were dull. The rear limbs were severely weak and even progressed to partial paresis. The activities of serum aspartate aminotransferase (AST), alanine transaminase (ALT) and alkaline phosphatase (AKP) and urea nitrogen (BUN) levels in the poisoned rabbits increased significantly. Serum α-mannosidase (AMA) activity decreased markedly. Pathomorphological lesions in the locoweed-poisoned rabbits developed severe microvacuolation of visceral and neurological tissue. Extensive vacuolation was observed in the liver, kidney and brain. These clinical and pathological features are similar to the symptoms of locoism.