This study was undertaken to search for a new antihypertensive or hypotensive agents in natural crude products in Korea. Betula schmidtii Regel, a tree belonging to the Betulaceae family, is cultivated for the most part of our country, and their cortecies or tree milk has been used as folk remedy, which exerted good antiinflammatory effect on rheumatic fever, arthritis or rheumatism. The rats were treated with several extracts from Betula schmidtii Regel, that is, extracts of water, methanol from inner cortecies, extracts of ether, methanol, hexane, chloroform or butanol from outer and total cortecies. In this experiment, we measured the blood pressure and heart rate in anesthetized rats, and atropine, diphenhydramine, phentolamine, propranolol, epinephrine, hexamethonium and hydralazine were pretreated prior to Betula extracts to clarify the mechanism of the hypotensive action. The results of the experiment were as follows; 1. BOIE(Betula outer cortex water-insoluble extract) and BICE(Betula inner cortex chloroform extract) elevated the blood pressure and heart rate. 2. BIME(Betula inner cortex methanol extract), BIBE(Betula inner cortex butanol extract) and BIWE (Betula inner cortex water extract) significantly lowered the blood pressure and heart rate. 3. With the pretreatment of hydralazine, the hypotensive effect of Betula schmidtii Regel was significantly blocked. From the above results, it is suggested that the hypotensive effect of Betula schmidtii Regel is due to the direct vasodilation of blood vessel.