Abstract: Toosendanin is a principle extracted from the bark of Melia toosendan Sieb et Zucc. It is used as an ascaricide.Some definite ultrastructural changes were observed in the nerve terminals of the diaphra gmatic neuromuscular junction after intramuscular injection of toosendanin in rats. The changes included a noticeable decrease in the number of synaptic vesicles and an increase in the number of elongated vesicles. Furthermore, it was found that the phagosome-like structure occured frequently and the mitochondria appeared to be more scattered.The results are discussed in relation to the possible mode of action of toosendanin.