The purpose of this paper is to examine the role of endothelium-derived relaxing factors in the control of coronary vascular resistance in conditioned subjects (i.e., after exercise training for a period of time sufficient to complete adaptation processes). Results from studies with exercise trained (EX) dogs, miniature swine, and rats are summarized. Since the relative importance of vascular control mechanisms differ in various segments of the coronary arterial tree, the effects of EX on conduit arteries and the coronary arterial microcirculation are discussed separately. Results indicate that endothelium-mediated vasodilator responses are normal in conduit coronary arteries of EX dogs, miniature swine, and rats. It is proposed that endothelium-mediated vasodilation of conduit coronary arteries is enhanced early in the exercise-adaptive process but returns to normal as adaptation to EX is complete, when structural adaptations produce a relative decrease in coronary shear during exercise. EX miniature swine manifest enhanced endothelium-mediated vasodilation stimulated by bradykinin and flow in isolated coronary resistance arteries and appear to have increased expression of NO synthase (ecNOS). Brief training also appears to increase the expression of ecNOS. The role of endothelium-mediated vasodilation in regulation of coronary blood flow in EX animals remains uncertain.