Reactive hyperemia following occlusions of 15, 30, 60 and 120 sec duration was studied in the left coronary vascular bed of the isolated fibrillating dog heart perfused with arterial blood at constant pressure or constant volume. Except for repayment, linear correlations were found between occlusion time and the characteristics of reactive hyperemia. At a basal perfusion pressure of 50 mm Hg the postocclusion reaction was absent. The maximum hyperemic response was observed at 100 mm Hg, while on a further increase of perfusion pressure reactive hyperemia decreased. The postocclusion reaction was more marked but of briefer duration under constant pressure perfusion. The results can be explained by the joint effects of metabolic, myogenic, and physical factors on coronary vascular tone.