The ability of external counterpulsation (Cardiassist) and intra-aortic balloon pumping (AVCO) to influence collateral coronary blood flow in ischemic myocardium was measured in anesthetized dogs. Cardiac output and heart rate (atrial pacing) were held constant on right-heart bypass. Both external counterpulsation and balloon pumping augmented peak diastolic pressure (30 mmHg and 38 mmHg, respectively), while mean aortic pressure, peak left-ventricular pressure, left-ventricular end-diastolic pressure, maximum left-ventricular dp/dt, hematocrit, and osmolality remained unchanged. Regional coronary blood flow was measured using 9-mum radioactive microspheres. External counterpulsation and balloon pumping begun immediately following ligation of the left-anterior descending coronary artery significantly increased collateral coronary blood flow 29 +/- 7.5% (SE, P is less than .01) and 20 +/- 8% (P is less than .05), respectively, to ischemic myocardium. This redistribution of collateral coronary blood flow produced by both methods of counterpulsation was primarily to the subepicardial region of the ischemic myocardium. The mechanism responsible for the measured increases in collateral coronary blood flow appears most likely to be an increased pressure gradient produced by diastolic augmentation.