Effects of 6 coronary vasodilators on the coronary blood flow and the contractile force of the ventricular muscle were examined simultaneously by injecting these drugs to the arterially blood-perfused canine papillary muscle preparation. All compounds produced a dose-dependent increase in blood flow rate, and relative potencies determined on the basis of doses producing a 100% increase in blood flow rate, ED100, were in the descending order : nifedipine greater than verapamil greater than diltiazem greater than dilazep greater than dipyridamole greater than carbochromen, and approximately 1 : 1/12 : 1/26 : 1/100 : 1/300 : 1/500. All drugs except for dipyridamole caused a dose-dependent decrease in the developed tension of the papillary muscle, although nifedipine and diltiazem in low doses produced a slight increase. Relative potencies determined on the basis of doses producing a 50% decrease in developed tension, ID50, were as follows: nifedipine (1), verapamil (1/13), diltiazem (1/40), dilazep (1/100), and carbochromen (1/270). Ratios of the ID50 to ED100 were as follows: diltiazem (5.2), nifedipine (3.5), verapamil (3.5), dilazep (2.5), and carbochromen (1.8). The higher the value the more predominant on the coronary vascular bed or the less depressant on the myocardial contractility were their actions.