This study was performed to determine whether nitroglycerin can increase blood flow to collateral-dependent myocardium during exercise. Intermittent repetitive occlusions of the left circumflex coronary artery (LCX) were used to stimulate growth of coronary collateral vessels in seven adult mongrel dogs. Coronary pressure distal to the occluder was measured with a long-term implanted microcatheter. When sufficient collateral growth had occurred to increase distal coronary pressure to >40 mm Hg during occlusion, the artery was permanently occluded. Dogs were returned to the laboratory 1 week later for study. Measurements were obtained at rest and during treadmill exercise during control conditions and after a 300-μg bolus of nitroglycerin (i.v.). Aortic and coronary pressures were measured with fluid-filled catheters, whereas myocardial blood flow was measured with radioactive microspheres. During control conditions, exercise caused significant increases of blood flow in the normal and collateral zones with significant decreases in vascular resistance. However, nitroglycerin failed to cause a further increase in blood flow to either the normal or the collateral-dependent myocardial regions during exercise. Furthermore, neither calculated transcollateral resistance (TCR) nor small-vessel resistance (SVR) changed significantly in response to nitroglycerin (TCR, 27 ± 9 mm Hg/ml/min/g before nitroglycerin and 27 ± 6 mm Hg/ml/min/g after; SVR, 43 ± 5 mm Hg/ml/min/g before nitroglycerin and 49 ± 7 mm Hg/ml/min/g after). The finding that the collateral vessels failed to dilate in response to nitroglycerin suggests that the nitric oxide system is already maximally recruited during exercise.