Few multiaxial constitutive laws under the vasoactive condition have been proposed as compared with those under the passive condition. The biaxial isometric properties of vasoactive rabbit arteries were studied, although the constitutive law was not proposed. The purpose of the present study is also to describe the multiaxial active mechanical properties of arteries. A novel strain energy function for the active stress has been proposed. This function is simple and may describe the multiaxial characteristics of constricted vessels. Although this study used mean stress and mean stretch ratio to determine the mechanical properties of vessels, a triaxial constitutive law of constricted vessels may be developed. There remains the subject of residual strains under active condition. If this problem will be solved, the accurate stress analysis under vasoactive conditions is possible.