Coptis rhizoma had been applied in the chines medicine as well as in the folk remidies and it was advocated that coptis rhizoma exerts good therapeutic effect in some case of alimentary disorders and other hypotensive ailments. However the basic pharmacology and the mechanism of hypotensive action are not clear. This experiment was carried out in order to reevaluate the pharmacology, especially hypotensive action of coptis rhizoma and to clarify the mechanism of action, in the rabbits and also LD₅₀ of coptis rhizoma was determined using mice as experimental animals. The results of the experiment were as follows. 1) LD₅₀ to the mice was 69 mg/kg. 2) Blood pressure manifested gradual responses by the fall of -17, -26, -62 mmHg in proportion to the administration of 3, 5, 10 mg/kg of coptis rhizoma extract respectively. 3) No significant differance was observed in heart rate between the normal control group and the coptis rhizoma extract treated group. 4) Administration of coptis rhizoma extract 5 mg/kg to the rabbit pretreated with atropine or avil did not show any significant difference in the blood pressure compared with that of control group. 5) In the rabbit pretreated with coptis rhizoma extract 5mg/kg, the hypotensive effect of epinephrine was significantly inhibited in comparison with that of normal rabbit.