The effects of intra-aortic balloon pumping (IABP) on myocardial flow distribution were studied in 50 dogs. Cardiac output was controlled by right heart bypass. In each dog the following parameters were measured with and without IABP during normal coronary perfusion and after regional ischemia was induced by anterior descending coronary vein flow by timed collection, and endocardial/epicardial flow ratios by a previously reported thermal washout technique. In nonischemic myocardium, IABP significantly (p less than 0.05) increased mean coronary sinus flow 11.5 percent +/- 5.8 percent (S.D.) and the mean endocardial/epicardial ratio, 17.3 percent +/- 0.28 percent. In the regionally ischemic myocardium, IABP significantly (p less than 0.05) increased mean segmental coronary vein flow 13.9 percent +/- 1.23 percent but decreased the endocardial/epicardial ratio 29.9 percent +/- 1.1 percent. We conclude that in the dog, IABP enhances subendocardial blood flow in perfused but not in ischemic myocardium. Contrary to common suppositions, the increase in collateral blood flow with IABP preferentially supplies epicardial layers in segmental ischemic zones, but may be shunted from the subendocardium.