The ventricular volume and function changes induced by the addition of 12 cm H2O of positive end-expiratory pressure (PEEP) during mechanical ventilation were studied in 11 patients with the adult respiratory distress syndrome. Cardiac output was measured by thermodilution and ventricular ejection fraction by the multiple gated equilibrated cardiac blood pool scintigraphy. Right and left end-diastolic volumes were then calculated by dividing stroke volume by ejection fraction. The PEEP caused a 14 percent decrease of the cardiac output secondary to a decrease in stroke volume. On the basis of the relationship between stroke volume and ventricular end-diastolic volume, we conclude that reduction in preload was the major component of the decrease in cardiac output. After removal of PEEP, we observed a rebound phenomenon characterized by higher values for stroke volume and cardiac output than before the application of PEEP.