13 subjects standing in a narrow upright tank were exposed to rapid water immersion up to the neck. The tank was filled from an elevated reservoir in less than 4 s. In 8 subjects heart size was measured by Roentgen-cinematography combined with video tape recording. Planimetry of the diastolic postero-anterior area of the heart showed an average increase in heart size of 30% within 6 s. In 5 subjects central venous pressure and the height of hydrostatic pressure in the tank were recorded. The two pressures rose and fell simultaneously without delay when the hydrostatic pressure exceeded the level of the diaphragm. Rapid immersion caused a fall in heart rate of approximately 20%. The findings speak in favor of the concept that the right heart does not constitute a resistance and that the pulmonary circulation and the systemic capacitance vessels form a functional unit.