1. Previous work has shown that physiological increases in mean aortic root pressure, which change the pressure in both the coronary circulation and the left ventricle, result in reflex vasodilatation. This study was undertaken to attempt to localize the reflexogenic area mainly responsible for the reflex. 2. In anaesthetized, artificially ventilated dogs, cannulae connected to perfusion systems were inserted in the ascending aorta, left ventricular apex and left atrium. This allowed us to change the pressures in: (a) the aortic root including both the coronary arteries and the left ventricle; (b) aortic root and coronary arteries, at constant ventricular pressure; and (c) in the ventricle, with mean (although not pulse) aortic pressure constant. Aortic and carotid baroreceptors were perfused at constant pressure and reflex responses were determined from changes in perfusion pressures (flows constant) to a vascularly isolated hindlimb and to the remainder of the systemic circulation. 3. Combined changes in mean aortic root (coronary arterial) and ventricular systolic pressures consistently resulted in decreases in perfusion pressures. A change in only mean aortic root (coronary arterial) pressure, with ventricular pressure constant, also resulted in decreases in perfusion pressures and these were only a little smaller than those to the combined stimulus. Changes in ventricular systolic pressure resulted in responses averaging only about 30% of those to the combined stimulus. 4. Setting mean aortic root or ventricular systolic pressures at different levels did not affect the responses to changes in pressures in the other region. 5. These results show that physiological increases in pressure in the aortic root and coronary arteries, in the absence of changes in pressure in the left ventricle, cause reflex vasodilatation. The relatively small response occurring when ventricular pressure was changed could be due either to a contribution from ventricular receptors or to a change in the stimulus to coronary receptors resulting from changes in the ventricular or aortic pulse. 6. We conclude that the reflex effects of increases in mean aortic root pressure are due mainly to stimulation of coronary arterial baroreceptors.