This study was performed to determine whether vasomotor tone of the coronary resistance vessels limits blood flow to ischaemic myocardium perfused by a stenotic coronary artery during exercise. Studies were performed on dogs in which a hydraulic occluder, in place for greater than or equal to 14 days, allowed production of a coronary stenosis, while distal coronary pressure was monitored with a miniature intra-arterial catheter. Treadmill exercise at 6.5 km h-1 with a 6% grade resulted in a mean heart rate of 209 +/- 4 beats min-1, with mean myocardial blood flow in the normally perfused left ventricular region of 2.90 +/- 0.37 ml min-1 g-1. An arterial stenosis that decreased coronary pressure to 40-42 mmHg resulted in a decrease of myocardial blood flow to 1.07 +/- 0.19 ml min-1 g-1 (P less than 0.01), hypoperfusion being most severe in the subendocardium. Intracoronary administration of the selective alpha 1-adrenergic antagonist, prazosin, resulted in a 50 +/- 14% increase in blood flow with no change in perfusion pressure. This increase in flow in response to prazosin was uniform across the left ventricular wall, from epicardium to endocardium. After administration of the selective alpha 2-adrenergic antagonist, idazoxan, there was a trend toward higher blood flow in the region of myocardium perfused by the stenotic coronary artery, but this change did not achieve statistical significance. These data indicate that residual vasomotor tone may limit blood flow to ischaemic areas of myocardium perfused by a stenotic coronary artery, and that this vasoconstriction is mediated, at least in part, by alpha 1-adrenergic mechanisms.