The present experiments have been conducted to study the immediate effects of graded immersion on the central circulation. When taking heart volume as an indicator, it was found that immersion to the diaphragm of a standing subject produces the same changes as assumption of the supine posture. Heart volume increases by approximately 130 ml. When the water level is raised to the neck, an extra pressure corresponding to a water column extending from the diaphragm to the surface of the water of approximately 25 cm H2O forces blood into the thorax. The heart becomes distended by an additional 120 ml. Correspondingly the central venous pressure at the height of the right atrium increases from 2.5 to 12.8 mm HG when the water level rises from the diaphragm to the neck. The greater filling of the pulmonary circulation is accompanied by a decrease in vital capacity and visualized by scintigrams. The preferential increase in blood volume of the apical regions is striking. When raising the water level from the symphysis to the xiphoid heart rate falls by about 15%.