Pressure measurements were made in the right atrium, vena cavae, and internal jugular vein in supine and erect positions in eight normal subjects and two patients with massive pericardial effusion. The pressures were identical at the above sites in supine position. On assumption of the erect posture, pressure decreased in veins superior to the heart and became negative (— 7 to —18 cm, average — 11 cm H2O) in the superior vena cava in subjects with essentially normal cardiovascular systems. A marked increase in pressure (9 to 21 cm, average 15 cm H2O) was observed in the internal jugular vein in these subjects just superior to the thoracic inlet above which pressure either remained close to atmospheric level or decreased by 1 cm H2O/cm vertical height. This venous pressure gradient across the thoracic inlet is due to potential collapse of the internal jugular vein which results from decrease in the transmural pressure of the vein in the neck in erect posture. No such gradient was observed in the two subjects with massive pericardial effusion when erect as the elevated venous pressure maintained positive transmural pressure throughout the venous system in all body positions.