The extract of an Oriental medicine "bushi", Aconitum japonicum roots (raw), was fractionated monitored by the analgesic activity in the tail pressure test, leading to the isolation of a mixture of the aconitines but no other constituents as the active principles. The aconitines have been compared in a variety of animal tests for analgesic activity. As the results, the alkaloids have proved in the tail pressure method and the acetic acid-induced writhing method to show analgesic activity in decreasing order : mesaconitine > aconitine > hypaconitine, though they exhibited no significant activity in the hot plate method. Heating of the raw Aconitum roots at 100°, which is one of the processing procedures of the crude drug, has brought about a rapid decrease of the analgesic activity with concomitant decrease of the content of the aconitines, demonstrating that the benzoylaconines produced by hydrolysis of the aconitines showed little or no analgesic activity. Assay of the benzoylaconines has revealed a weak analgesic activity in the acetic acid-induced writhing test. On the basis of the present results, it is predicted that the clinical effectiveness of the crude drug is due to the aconitines (especially mesaconitine) and partly to the benzoylaconines.