The isolated right atrium of the dog heart was perfused with arterial blood introduced from a carotid artery of a support dog. The selective administration of pentobarbital into the cannulated sinus node artery induced a negative inotropic effect without any significant change in chronotropism at relatively small doses. At higher doses, pentobarbital caused both negative chronotropic and inotropic effects. Even at that time, the negative chronotropic response to pentobarbital was much less than the negative inotropic response. Accordingly, pentobarbital could show a dissociation of electrical and mechanical activity in this preparation. On the other hand, either verapamil or manganese caused a negative chronotropic effect at the threshold dose for inducing a negative inotropic effect. It is suggested that the mode of cardiac action of pentobarbital may differ from calcium antagonists such as manganese or verapamil, although pentobarbital-induced effects may be associated with an impaired removal of calcium ion.