The effects of changes in cardiac rhythm or conduction of the rate of volume flow in the superior vena cava (SVC) were studied in 25 dogs anesthetized with sodium pentobarbital. Slowing of sinus rate from 145 ± 16 to 114 ± 12 beats/min was achieved by selective perfusion of Ringer's solution into the sinus node artery. During sinus slowing there was a continuous change in the systolic-diastolic flow ratio measured in the SVC. At control sinus rate (net), total forward flow as 6.3 ml/beat with a systolic volume of 6.5 ml and a diastolic volume of -0.2 ml. At the slowest bradycardia, total forward flow had increased by 25%. Since there was no significant change in the systolic volume, the entire 25% increase in forward flow occurred during ventricular diastole. During atrioventricular (AV) block (second-degree or complete), achieved by selective perfusion of eserine into the AV node artery, unimpaired acceleration of flow was readily demonstrable in the SVC even in the absence of ventricular contraction. Thus systolic ventricular suction (vis a fronte) has little or no effect on right atrial filling. During high-degree AV block, the normal atrial contraction readily opened the tricuspid valve, and when both atrium and ventricle were filled to or near maximum capacity, atrial contraction was consistently vigorous enough to propel blood in to the pulmonary artery. Given appropriate hemodynamic circumstances, these results confirm the remarkable effectiveness of right atrial contractile performance.