Abstract: The LD₅₀ in mice was found to be 19.4 g/kg and 0.98±0.075 g/kg respectively for the infusion and crude extract of Chou-wu-tung, given by intravenous injections. The subacute toxicity is negligible when the infusion was crally administered to rats (0.25—2.5 g/kg/day) for 60 days, except that some animals showed quietness, slight fall of systolic pressure and soft stool. No apparent toxic effects were observed. A crude extract and infusion of Chou-wu-tung, when injected intravenously at respective dosages of 50—100 mg/kg and 150 mg/kg, produced primary and secondary falls of blood pressure on anaesthetized rats and dogs, but with intramuscular injection or oral administration, only the secondary fall of blood pressure was apparent. This hypotensive effect might last for 2—3 hours. Intravenous injection of the decoction to anaesthetized animals produced only primary fall of blood pressure, the infusions of alcohol, ether and ,chloroform, given by the same route, did not lower the blood pressure. Twenty-eight renal hypertensive rats were divided into three groups, the first (11 rats) and the second (7 rats) groups were treated with the infusion at respective dosages of 0.5 g/kg/day and 5 g/kg/day, and the third (10 rats) with the crude extract at 50 mg/kg/day. The drug was given to all the rats for 2 weeks by oral administration. Hypotensive effect was observed in 3—10 days after medication, and became more remarkable in the second week of the therapeutic period or the first week after cessation of medication; the blood pressure dropped 57.4%. The blood pressure in most rats returned in 2 weeks, and in some rats in 2—4 days or 4 weeks after the stopping of medication.