Since it was proposed that vanadate may be an ‘ideal endogenous regulator of the Na⁺, K⁺-ATPase activity (Cantley et at, 1979), vanadate has been a subject of intensive research and a variety of its physiological effects have been described (Nechay, 1984). In isolated guinea pig heart muscle vanadate shows a positive inotropic effect on ventricular muscle, while it induces a negative inotropic effect on atrial muscle. But its underlying mechanism has not been elucidated so far. Therefore, in this study the flux rates of calcium ion into and from guinea pig heart muscle were measured to throw some light on the underlying mechanism, because those rates have been known to be closely related to the cardiac contractility and the results are summarized as follows: 1) Calcium efflux rates from the intracellular Ca⁺⁺ pool (compartment 4) of both guinea pig left atrium and right ventricle were significantly reduced by vanadate and their pool sizes were significantly increased by vanadate. 2) The magnitude of calcium influx into left atrium was reduced by vanadate, While the magnitude of calcium influx into right ventricle was not affected by vanadate. From these results, it may be concluded that the positive inotropic effect of vanadate on the ventricular muscle was due to a reduced efflux rate of calcium ion and its negative inotropic effect on atrial muscle was resulted from a reduced influx of calcium ion.