Abstract: Rotundine is one of the alkaloids isolated from the root of Stephania rotunda Lout. Its chemical structure is similar to tetrahydropalmatine (THP) and tetrabenazine (Fig. 1). It has been reported that tetrabenazine possessed the same mechanism of sedative-tranquilization as reserpine. Our previous results showed that the mechanism of sedative-tranquilizing action of THP was different from that of reserpine. It is interesting to compare some neuropharmacological actions of rotundine with l-THP and reserpine. The results are as follows: (1) In mice intraperitoneal injections of 20—40 mg/kg of rotundine exhibited a marked sedative-tranquilization. Large doses of this drug also caused an obvious ataxia. Intravenous injection of rotundine 15 mg/kg to rabbits produced an analgesic effect. Under the influence of large doses (40 mg/kg) rabbits developed catatonia-like symptoms. (2) In mice or rabbits pretreated with rotundine or l-THP, the persistent sedativetranquilization of reserpine could not be altered. Premedication with β-phenyl-ethylhydrazine (MAOI) brought about the reserpine-reversal, but did not reverse the action of rotundine or l-THP. The reserpine reversal excitation, however, could be antagonized by rotundine as well as by l-THP. Rotundine and dl-THP did not decrease the brain 5-HT level, but reserpine gave such effect. (3) During stimulating the contralateral super-radial nerve in curarized cats, the evoked potentials were simultaneously recorded from midbrain reticular formation and hypothalamus. Intravenous administrations of rotundine or l-THP (10—20 mg/kg) revealed a decrease in the amplitude of the evoked potentials of both areas. After the injection of rotundine or l-THP to rabbits or cats the synchronization of spontaneous EEG was often observed both in cortical and subcortical areas. From the above-mentioned data, it may be considered that the mechanism of sedative-tranquilization of rotundine is similar to that of l-THP rather than to that of reserpine-tetrabenazine type compounds.