To study the relative importance of cardiac and peripheral effector mechanisms in the carotid sinus baro-reflex in man cardiovascular responses to equal changes of the carotid sinus transmural pressure (Ptm) in either direction of the normal were recorded and compared in eight physically well-trained young male volunteers. In both the supine and the 70 degrees head-up position, a decrease in Ptm produced a more potent reflex response of the systemic arterial pressure than did a similar increase in Ptm. Whereas the arterial pressure response to increased Ptm was due solely to a reduction in vascular resistance, a significant increase in cardiac output contributed to the more potent pressor response to a decrease in Ptm and thus to the predominantly antihypotensive properties that characterize the carotid sinus baroreceptor control system in man. However, since combined beta-adrenergic and parasympathetic blockade abolished the effect of reduced Ptm on cardiac output without greatly impairing the blood pressure response, it is concluded that adjustments in cardiac output are not of critical importance in the buffering function of the carotid sinus baroreceptors. Autonomic cardiac blockade exaggerated the fall in cardiac output on head-up tilt, the arterial pressure remaining unaffected due to a compensatory increase in systemic vascular resistance.