An objective method for the quantitative measurement of the coronary blood flow in ml./min. using a coincidence counting system with rubidium-84, a positron emitter, is described. Accidental coincidence counts arising from independent annihilation of positrons were greatly reduced by simple shielding.The coronary blood flow was calculated by the extrapolation of the clearance values to zero time. At this time the myocardial extraction ratio of rubidium-84 is unity and clearance is equal to flow. The accuracy of thismethod was demonstrated on the isolated perfused dog heart.It was found that the rate of change of the extraction ratio of rubidium-84 in the isolated perfused dog heart and in man was not altered with changes in flow.The finding that nitroglycerin increases the coronary blood flow in patients without coronary artery disease while decreasing it in patients with coronary artery disease was confirmed.