Since early time the bark and fruit of some species of plants belonging to family melia have been used as ascaris vermifuge in traditional Chinese medicine. In our previous studies it was shown that toosendanin, a triterpenoid derivative extracted from the bark of Meli toosendan Seib et Zucc was a neuromuscular blocker acting selectively and irreversibly on the release of acetylcholine from the nerve terminals. It was thus suggested that toosendanin binding sites should exist in presynaptic nerve terminals. The binding experiments of 3H-toosendanin to the pellet of rat cerebral cortex homogenate demonstrated that the binding site of 3H-toosendanin exists in the cortex homogenate. The equilibrium dissociation constant (KD) of toosendanin-binding site complex is about 2.0 x 10(-7) mol/L. The specific binding activity of the homogenate pellet estimated according to the binding data is about 2.7 x 10(-12) mol per mg protein. The Hill coefficient of this binding site is 0.97. Our data also indicated that the synaptosomes prepared by sucrose density gradient ultracentrifugation are enriched with components of toosendanin binding site.