Myocardial blood flow was studied with radioactive microspheres in 14 dogs 1 mo after placement of an Ameroid constrictor on either the left circumflex or the left anterior descending coronary artery to result in total coronary occlusion without myocardial infarction, as well as in 7 normal control dogs. Measurements were performed during quiet resting conditions and during two levels of treadmill exercise to achieve heart rates of approximately 180 (light exercise) and 230 beats/min (heavy exercise). During resting conditions myocardial blood flow in the collateral-dependent area was normal in all dogs. Three different patterns of response occurred during treadmill exercise in the animals with coronary occlusion. In five dogs, both the volume and transmural distribution of myocardial blood flow behaved similarly to the normal dogs at both exercise levels. In five dogs, blood flow behaved normally during light exercise, but during heavy exercise a transmural redistribution of perfusion occurred in the collateralized area to result in subendocardial underperfusion. Finally, in four dogs even light exercise resulted in subendocardial underperfusion in the collateral-dependent area, whereas further increasing the exercise load resulted in an actual decrease of subendocardial blood flow below the resting level. These data demonstrated that although resting myocardial blood flow was normal in the collateral-dependent area 1 mo after Ameroid implantation, a markedly heterogeneous response occurred during exercise, ranging from a completely normal response to a severe degree of exercise-induced subendocardial underperfusion.