Volume-pressure curves were determined for the living left heart vascular segment by injecting fluid into the left atrium during momentary cardiac asystole and measuring left atrial pressure. Such curves show 2 essentially linear relationships with a break at a low pressure level. It has been suggested that this break is due to closure of the pulmonary veins or the pulmonary-left atrial junction at low pressure levels. This closure prevents regurgitation into the pulmonary veins. Higher atrial pressures overcome this block and permit retrograde perfusion of the pulmonary pathways. This suggestion was tested by measuring left atrial and pulmonary vein pressure during the injection of blood into the atrium of the quiescent heart. The results are consistent with such a collapse of the veno-left atrial junction at low pressure levels.