The hemodynamic effects of dipyridamole were evaluated in 25 subjects without evidence of coronary arterial disease. The method used involved estimation of myocardial Rb 86 clearance with simultaneous recording of radiocardiographic data. In this way, changes in myocardial perfusion due to the drug could be interpreted together with concurrent hemodynamic changes in the systemic and pulmonary circulation. Dipyridamole was administered as a single intravenous dose of 10 mg. to 6 subjects and 20 mg. to 7 subjects. Determinations were made both before and 3 minutes after administration. In another 12 subjects determinations were made before and 10 minutes after an intravenous dose of 20 mg. Dipyridamole produced a marked increase of myocardial clearance in the majority of these subjects. Three minutes after injection the observed increase of myocardial clearance was associated with an increase of cardiac output and left ventricular work and a reduction of peripheral resistance. Ten minutes after injection the effect on the coronary circulation was still present while the other changes tended to disappear. Thus, the best relationship between myocardial perfusion and cardiac work was found 10 minutes after the administration of the drug.