The hemodynamic manifestations of the Valsalva maneuver are in part the result of changes in the venous return accompanying changes in intrathoracic pressure. Doppler echocardiography was performed during Valsalva maneuver in 13 normal subjects. Superior vena cava flow velocities and flow velocity integrals were measured in all 13 subjects. In the 5 subjects in whom the superior vena cava was clearly visualized throughout the maneuver, vena cava diameter was also analyzed. The superior vena cava flow velocity integral at rest was 17 +/- 2 cm. It diminished significantly, disappeared or reversed (-13 +/- 6 cm, p less than 0.001) with phase I of the maneuver. During the maintenance phase (phase II), the flow velocity integral increased significantly (31 +/- 2 cm, p = 0.05 vs baseline and phase I) and was associated with a decrease in superior vena cava lumen diameter at the time of Valsalva and continuing throughout the strain. With release of the maneuver (phase III), there was a sudden significant increase in flow velocity integral (61 +/- 2 cm, p = 0.005 vs phase II) and superior vena cava lumen diameter. Subsequently, superior vena cava flow velocity integral returned to baseline values. This study suggests that one of the ways in which the Valsalva maneuver leads to decreased venous return may be by direct external compression of the superior vena cava.