This study was designed to evaluate the effect of nitroglycerin (30 micrograms given as an i.v. bolus) on regional distribution of myocardial blood flow in conscious dogs, following an acute coronary occlusion similar to a coronary spasm. The left anterior descending (LAD) coronary artery was acutely occluded with a balloon cuff occluder. The distribution of blood flow between the endocardium and the epicardium of both the normal and ischemic area of the left ventricle was determined by means of the radioactive microsphere technique. Acute occlusion of the left anterior descending coronary artery produced a significant decrease of blood flow reaching the area irrigated by this artery; this decrease was of a lesser magnitude after administration of nitroglycerin. In addition, ischemia produced a disproportionate decrease in endocardial blood flow. This decrement was also of a lesser magnitude following administration of nitroglycerin. Blood perfusion to the non-ischemic myocardium was not altered. These results indicate that an intravenous bolus of nitroglycerin, given after a brief coronary occlusion simulating a coronary spasm, increases blood flow to the ischemic myocardium, induces a favorable redistribution of blood flow toward the ischemic endocardium and does not produce any decrement of blood perfusion to the non-ischemic myocardium.