In several previous papers1 we have reported studies on the cardiac output of patients with congestive heart failure. Such persons usually have subnormal values for the circulatory minute volume, but no correlation between the amount of blood pumped by the heart and the clinical state of the patient was observed. It was noted that a disappearance of congestive phenomena might be associated with an increase, a decrease or no change in the cardiac output. These observations have led to the conclusion that the theory of forward failure does not offer an adequate explanation for the mechanism of congestive heart failure. Our previous studies have not been directly concerned with the relation of the congestive syndrome to back pressure. However, as has been pointed out,1a the observations reported are compatible with this idea as originally expressed by Hope.2 One of the chief points of the back pressure theory