A comparative study was made between the cardiac action of diethyl 4-(benzothiazol-2-yl)benzylphosphonate (KB-944) and those of nifedipine, diltiazem and verapamil in anesthetized dogs and isolated right atria of guinea pigs. In anesthetized dogs, KB-944 caused a dose-dependent fall in mean blood pressure and dose-dependent increase in coronary sinus outflow. Coronary sinus outflow oxygen pressure was increased with increasing coronary sinus outflow; and consequently, the coronary arterio-venous oxygen difference was decreased. The cardiac output was slightly increased, while the myocardial oxygen consumption and myocardial work were reduced. In the isolated right atria of guinea pigs, KB-944 produced a decrease in spontaneous atria rate, and at the concentration of 10(-6) g/ml atrial standstill was seen in few preparations. The contractile force was decreased by KB-944. Especially, KB-944 exerted the weakest effect on contractile force among the tested drugs. These actions of KB-944 are very similar to those of nifedipine, diltiazem and verapamil.