Scutellariae Radix and its main constituent, baicalin, concentration-dependently contracted the isolated rat aorta, and its contractile responses were tachyphylaxic. Wogonin, another constituent in Scutellariae Radix, conversely concentration-dependently relaxed the contraction induced by phenylephrine in the isolated rat aorta, and the relaxation was not tachyphylaxic. Applying the sample containing baicalin and wogonin in the same proportion as the content ratio in Scutellariae Radix, it was observed that the isolate rat aorta contracted in the same way as Scutellariae Radix and baicalin. From the results with adrenergic blockers, serotonergic blockers, tetrodotoxine and NO synthase inhibitors, it is suggested that the mechanism of contractile effect of Scutellariae Radix and baicalin on the vascular smooth muscle is not based on the adrenergic and serotonergic system, and endothelium derived relaxing factor. Furthermore, cyclooxygenase inhibitors suppressed the contractile response to baicalin in the isolated rat aorta, correlation was identified between prostanoids concentration and the contraction by baicalin. Repeated applications of baicalin induced the decrease of prostanoids production while weakening the strength of contraction. The above results suggest that prostanoids production is involved in the contractile response to baicalin.