The ability of coronary collateral vessels to supply an adequate volume of blood to the subendocardium during the resting state and during periods of stress was studied. Regional myocardial blood flow was determined by the radioactive microsphere technique. Ameroid constricting devices were placed around the left circumflex coronary artery in 22 adult mongrel dogs. Three months later, regional myocardial blood flow was measured in a normal area of the left ventricle as well as in the area supplied entirely by coronary collateral vessels. Collateral blood flow was sufficient in the resting state to prevent myocardial ischemia distal to a gradually occluded coronary artery. However, in 10 animals (group 1) without aorta coronary bypass grafts (ACBG), a selective underperfusion of the collateralized subendocardium occurred during periods of stress. In 12 animals (group II), this selective underperfusion of the collateralized subendocardium with stress was abolished by placing ACBG's distal to the site of coronary artery occlusion.