Right atrial pressure (RAP) may become substantially elevated during heart failure and has been reported to reduce collateral flow to the ischemic myocardium of isolated hearts. The effect of elevated RAP on blood flow to collateral-dependent and normal myocardium of in situ hearts was studied in 20 open-chest anesthetized dogs with acute occlusion of the left anterior descending coronary artery. Regional myocardial blood flow was measured with radioactive microspheres while RAP was elevated by restricting right ventricular (RV) outflow with constant aortic pressure. Increasing RAP from 3.8 ± 0.5 to 21.5 ± 0.8 and then to 34.3 ± 0.9 mmHg did not reduce blood flow to any transmural region of LV normal or collateral-dependent myocardium. Further elevation of RAP to 49.3 ± 1.1 mmHg reduced subepicardial but not subendocardial collateral flow. Blood flow to normal RV increased. Retrograde flow and peripheral coronary pressure increased as RAP was elevated. Previously injected 11-µm microspheres were present in the retrograde flow when RAP was elevated; thus retrograde capillary flow contributed to the retrograde flow. The results explain discrepancies among previous reports, and they are consistent with a waterfall phenomenon in the coronary collateral circulation.