The coronary circulation of anesthetized dogs was tested for the presence of vascular waterfalls by manipulating coronary arterial and coronary venous pressures. The left main coronary artery and the coronary sinus were cannulated, and relationships between coronary artery pressure, coronary sinus pressure, and coronary flow were studied. Experiments were conducted during diastolic arrests, under steady state conditions, in the absence of autoregulation. Relations of coronary flow to coronary sinus pressure at constant coronary artery pressure were consistent with the presence of a vascular waterfall in the coronary sinus. When the great cardiac vein was cannulated, relations of great vein flow to great vein pressure at constant coronary artery pressure were consistent with the presence of a vascular waterfall in the great vein, indicating that waterfall behavior can occur in epicardial veins other than the coronary sinus. In dogs on right heart bypass, with the coronary sinus and great vein uncannulated, the relationship between right atrial pressure and coronary sinus pressure showed a waterfall pattern, indicating that the waterfall is not an artifact of venous cannulation. In the right heart bypass experiments, venous waterfall behavior was seen in beating hearts as well as during diastolic arrests. We conclude that a vascular waterfall is present in epicardial coronary veins which can significantly influence coronary blood flow.