The effects of intravenous administration of nitroglycerin or dipyridamole on distribution of myocardial blood flow in the heart after coronary underperfusion were investigated. In 18 open chest, anesthetized dogs, flow of the tubing supplying the left circumflex coronary artery (LCA) was reduced and kept constant, and then the drug was given intravenously. Before and after an administration of saline, nitroglycerin (0.04 mg/kg) and dipyridamole (0.2 mg/kg), myocardial blood flow was measured by tracer microspheres (TM). The first and second carbonized 15 mu spheres (TM1 and TM2) were injected into the left atrium and the cannulated LCA perfusion system respectively before medication. Similarly, the third and fourth spheres were given after drug. Thus, by using four different TMs (141Ce, 51Cr, 85Sr, 46Sc) regional myocardial blood flow from the LCA perfusion route (TM2 and TM4) was measured separately from that of the other normally perfused coronary route (TM1 and TM3). The effect of drugs on flow distribution of these two independent routes was measured. The LCA underperfusion produced a significant diminution of regional flow and its end/epi ratio in the LCA territory. Nitroglycerin increased end/epi ratio in the ischemic myocardium in the LCA perfusion route, but dipyridamole increased the normally perfused coronary flow but caused a shift of flow from the ischemic left ventricle to the atria and right ventricle within the LCA territory.