Abstract Electrocardiographic changes occurred during angiographic injection of the internal mammary arteries in dogs with pericardial splenosis and previously ligated coronary arteries. These electrocardiographic changes appear to be the result of the radiopaque material reaching the myocardium directly from the internal mammary arteries through the pericardial splenosis, as demonstrated by the short interval of time between the start of the injection and the onset of the electrocardiographic changes. The dramatic electrocardiographic changes, with timing measurements correlated with the simultaneous angiograms, support the validity of such conclusions in 2 of 6 test animals. No such electrocardiographic changes occurred with angiographic injection in 3 control dogs and in 4 other test dogs similarly injected. The India ink injections indicate anatomic pathways from the internal mammary arteries through the pericardial splenosis into the coronary arterial system within the myocardium. The electrocardiographic changes indicate an unmeasured but apparently significant quantity of collateral flow. If such pathways can develop, the problem then appears to be one of producing profuse splenotic growth more regularly.