The neuromuscular blocking actions of several constituents extracted from Japanese-sino medicine, aconite, were compared in mouse phrenic nerve-diaphragm muscle preparations. Hypaconitine (HAT) was more potent than aconitine (ATN), mesaconitine (MAT) and deoxyaconitine. Lipohypaconitine, coryneine and lipodeoxyaconitine were less effective. Lipoaconitine, benzoylmesaconine, higenamine, kobusine and chasmanine were not effective. The blockades by HAT, ATN and MAT were not recovered by neostigmine. The mechanisms of blockade were similar to that of aconite crude extract. These results suggest that aconite action is dependent on HAT, a main constituent.