Influence of the carotid sinus reflex on mean circulatory pressure was examined in anesthetized dogs with the vagus nerves intact. The carotid sinus reflex was elicited either by occlusion of the bilateral common carotid arteries (9 dogs) or by lowering intrasinus pressure of the isolated carotid sinuses (1 dog). When the reflex rise of arterial pressure was greater than 10mmHg, there also was an increase in mean circulatory pressure up to 2mmHg in 11 out of 20 instances. The increase in mean circulatory pressure tended to be greater as the reflex rise of mean arterial pressure was greater. Thus, when the pressor response was greater than 30mmHg, elevation of mean circulatory pressure was observed in 7 out of 8 cases. It is quite plausible that the control of cardiac output by the carotid sinus reflex is attributable, at least partly, to the Frank-Starling mechanism, since a reflex change in mean circulatory pressure will alter the venous return to the heart.