Studies are presented which indicate that the neural components of an atrio-renal reflex appear to be present in the primate and thus presumably in man. However, this reflex does not appear to contribute importantly to blood volume homeostasis in the primate. It is our hypothesis that it is the high-pressure baroreceptors, i.e., those in the carotid sinus and those in the aortic arch which play the major role in the neural control of blood volume in the primate and thus in man. This apparent evolutionary change in the importance of high pressure vs. low pressure receptors in the neural modulation of blood volume may be related to the assumption of an upright or semi-upright posture.