The exact microvascular site of coronary vasodilation after coronary artery occlusion has not been clearly established. We sought to determine 1) the microvascular site of recruitable vasodilator reserve after a critical stenosis as assessed by adenosine and EDTA, 2) the coronary microvascular site responsible for vasodilatation after total coronary artery occlusion, and 3) the microvascular site for recruitable vasodilator reserve after coronary artery occlusion as assessed by adenosine and EDTA. Hemodynamics and coronary epicardial microvascular diameter were measured in 33 dogs by means of intravital epiillumination microscopy at control conditions, during a critical stenosis, or 30, 60, and 120 min following coronary artery occlusion. To evaluate the site of pharmacologically recruitable vasodilator reserve, EDTA or adenosine was suffused onto the epicardial surface. After a critical stenosis EDTA dilated all sizes of microvessels, whereas adenosine only dilated small microvessels (less than 150 microns). After coronary artery occlusion there was vasodilation that was inversely proportional to the control microvascular diameter. Coronary microvascular diameter did not change during 2 h after coronary artery occlusion. In addition, intravenous or topically applied adenosine or topically applied EDTA had no additional effect on the coronary arteriolar diameter. We concluded that in response to sudden coronary artery occlusion, the coronary microvascular dilation was inversely related to the control microvascular diameter, and in contrast to the response after a critical stenosis, there was no pharmacologically recruitable vasodilator reserve to adenosine or EDTA following acute coronary artery occlusion.