Retrograde coronary flow (Qr) was collected from the cannulated distal bed of the ligated left anterior descending artery in 10 open-chest canine preparations. Mean systolic and diastolic aortic root pressure (MSP, MDP [mm Hg]), and heart rate were independently varied, employing intra-aortic balloon counterpulsation and atrial pacing. Intervention with isoproterenol and with nitroglycerin was studied. Qr increased with MDP in direct proportion to the function (0·006 MDP+0·00004 MDP2). Qr was not influenced significantly by MSP (-0·0002 MSP +1·02). Tachycardia resulted in decreased Qr, the reduction correlating with a decrease in the diastolic fraction (fd) of the cardiac cycle. Isoproterenol (two experiments) and nitroglycerin (two experiments) were slight collateral dilators. The results indicate that post-occlusion coronary collateral flow occurs almost entirely in diastole, and is determined directly by MDP and fd. The diastolic collateral flow is influenced by alterations in left ventricular end-diastolic pressure and only slightly by pharmacological interventions. The relative indifference of Qr to alterations in peak systolic pressure and heart rate suggests that coronary collateral flow is not primarily metabolically regulated in the acute post-occlusive state. A linear elastic model of the collateral bed is consistent with the experimental findings and lends to their interpretation.