Hemodynamics of instrumented beagles with 75-85% stenoses of the left circumflex (LCf) coronary artery were evaluated before and during transient LCf balloon occlusion at rest and during treadmill running. Studies were done after surgery and 3 mo later. Myocardial blood flow was measured with radioactive microspheres during the LCf occlusions. Nine dogs (group I) had patent LCf arteries at the second study, and six (group II) had occluded vessels. Transient LCf occlusion during running in group I at weeks 1 and 12 and in group II at week 1 caused significant left ventricular failure, whereas the same protocol in group II dogs at week 12 produced minimal hemodynamic changes. Left ventricular failure precipitated by coronary occlusion during exercise diminished from week 1-12 in those animals with increasing LCf collateral flow, but actually increased in the animals with diminishing collateral flow. In group II dogs after chronic LCf occlusion total flow to LCf myocardium as well as inner-to-outer layer ratios were normal both at rest and during exercise. Thus coronary collaterals do preserve myocardial function, limit adverse hemodynamic responses to myocardial ischemia, and restore postocclusion tissue flows to normal even during the stress of exercise.