The effect of ethyl adenosine-5'-carboxylate hydrochloride (EA) was determined in 17 dogs by the radioactive microsphere technique. In both experimental and control animals, microspheres were injected 5 min before (preocclusion period), 60 min after (occlusion period), and 75 min after (saline or vasodilation period) ligation of the left anterior descending coronary artery (LAD). Ten seconds prior to the third microsphere injection, saline was administered to five control animals, and saline with 0.5 mg/kg of EA was given to 12 experimental animals. There was no significant change in myocardial blood flow after saline injection. After EA, mean arterial pressure fell 10% (P less than .001); myocardial blood flow increased 275% (P less than .001) in the nonischemic left circumflex artery (CIRC) area and increased 82% (P less than .01) in the occluded LAD area. In addition, those animals with extensive collateral development showed a significantly greater increase in collateral blood flow than those with minimal collateral development (P less than .05). Thus, EA may increase flow to ischemic areas of myocardium dependent upon the extent of preexistent collateral supply.