Abstract: The present report is concerned with the hypotensive effect of the bark of paeony root and its volatile constituent,paeonol.The decoction of the bark,whether containing paeonol or after its having been extracted with ether,has been shown to produce a decrease of the blood pres- sure of anesthetized normal dogs and unanesthetized hypertensive dogs. One hypertensive dog given paeonol prepared by steam distillation of the bark showed remark- able fall of blood pressure.In the 4 hypertensive dogs given paeonol prepared by chemical synthesis 2 showed significant drop,1 showed some drop,and 1 showed no change in blood pres- sure.There were no abnormal findings in the electrocardiogram,blood picture,liver and kidney functions. Six of seven hypertensive rats given synthetic paeonol also showed remarkable fall of blood pressure.The LD₅₀ of paeonot in mice has been estimated to be 4.9±0.47 g/kg.