The hemodynamic effects of intermittent positive pressure ventilation were studied in lightly anesthetized dogs following recovery from implantation of pulsed ultrasonic flow transducers on the aorta and vena cava. A partial rebreathing system was utilized to maintain constant PaCO. Data were obtained during spontaneous respiration and using a respirator, with peak airway pressures of 10, 20 and 30 cm. of water, inspiratory to expiratory ratios of 1:2, 1:1 and 2:1. Maximum values for stroke volume and cardiac output occurred during spontaneous breathing. Cardiac output and aortic stroke volume decreased with increasing airway pressure and increasing inspiratory to expiratory ratios. Venous return was inhibited by increasing pressure, and changes in vena caval flow were reflected in changes in aortic flow within the time of two heart beats. The circulatory effects of positive pressure breathing are related to the mean intrathoracic pressure and the effect on venous return.