During reactive hyperemia following a brief coronary artery occlusion, excess arterial inflow exceeds the blood flow debt incurred during occlusion by 300-600%. The present study was performed to determine whether this marked reactive hyperemia is essential for restoration of coronary vascular tone. Coronary blood flow was measured in unanesthetized dogs with electromagnetic flowmeters and hydraulic occluders chronically implanted on their left circumflex coronary arteries. When 10 sec coronary artery occlusions were performed in pairs separated by a brief interval during which excess arterial inflow equaled the blood flow debt incurred during the first occlusion, reactive hyperemia following the second occlusion was no greater than that following an isolated control occlusion. Thus, approximately 100% repayment of the blood flow debt resulted in restoration of normal reactivity to a second occlusion. To further ascertain whether coronary vascular tone could be regained without the usual excess inflow, the reactive hyperemia following a 10 sec occlusion was mechanically limited by partial inflation of the occluder. When reactive hyperemia equal to 115±10% repayment of the blood flow debt was allowed, final complete release of the occluder was followed by no additional hyperemia. These studies demonstrated that the markedly excess arterial inflow which occurs during coronary reactive hyperemia is not essential for restoration of coronary vascular tone. (19 references).