Hemodynamic response to graded immersion was studied in healthy male subjects in a thermoneutral bath in the sitting position. Pressures in the right heart and cardiac output were determined by means of a semifloating catheter with a thermistor probe. Pressures in the right atrium, pulmonary artery and in pulmonary wedge position increased with increasing depth of immersion, cardiac output was likewise augmented. Heart rate decreased from rest to hip immersion but remained constant from hip to head out water immersion. Plasma norepinephrine concentration remained constant throughout the experiment. The reported changes depend on the blood shift from capacitance vessels into the thoracic cavity. From this, preload increased and cardiac performance was improved. However, in patients with disturbed left ventricular function, immersion to the neck may be potentially hazardous due to augmented left ventricular filling pressure.