We have isolated a sesquiterpene lactone, cumambrin A from the dried flowers of Chrysanthemum boreale Makino and reported its chemical structure. The aim of the present study was to investigate whether the exogenous administration of cumambrin A has a pharmacological effect on normalization of blood pressure in the spontaneously hypertensive rats (SHR). In vitro studies: Relaxative response induced by cumainbrin A was increased with dose-dependent manner and showed maximizing response at a concentration of $5{ imes}10^{-4}M$. Further, this relaxative response was significantly increased at a condition of endothelium present than that of endothelium denuded. In vivo studies: The normalizing effect of cumambrin A on blood pressure was also increased with time-dependent manner and then gradually recovered to normal condition at approximately 4 hrs after cumambrin A treatment.