Studies were conducted in 12 conscious, unsedated dogs instrumented for measurements of subendocardial segment length in the area perfused by the left circumflex coronary artery, blood flow velocity of this artery, and left ventricular pressure. An externally inflatable pneumatic occluder was placed to occlude the circumflex coronary artery for selected durations at selected intervals. Resting coronary blood flow velocity was measured before and after collateral development induced by repeated brief coronary occlusion. Under conditions of comparable myocardial oxygen consumption, shown by the similar pressure-rate product, preocclusion resting coronary blood flow velocity of the repeatedly occluded artery decreased by 21.3% (P less than 0.001) in association with collateral growth. These findings strongly suggest that the perfusion territory of the collateralized artery decreases considerably secondary to the periodic ischemic stimulus.