1. Experiments with vascularly isolated, blood-perfused aortic arch and carotid sinus preparations in sixteen dogs have provided evidence which suggests that, in the reflex regulation of normal arterial blood pressure, the aortic and carotid baroreflexes are not equivalent.2. Two different techniques were used. In one, a steel cannula was inserted into the ascending aorta and arch and fixed in position by ligatures about the aorta. The blood-filled space (aortic jacket) thus created could be distended with known pressures; the cardiac output passed through the cannula into the descending aorta. In the other, an extracorporeal circulation utilizing an isolated heart-lung preparation was used to separately perfuse the carotid sinuses, aortic arch, and systemic circulation of a test dog.3. Independent open-loop analysis of the aortic and carotid baroreflexes in each dog indicated that they were essentially similar in their over-all modus operandi but that there were quantitative differences between them which would suggest a predominant role for the carotid sinus reflex in the control of normal blood pressure.4. The carotid sinus Blutdruck-charakteristik curve was symmetrical about the range of normal blood pressure for the dog while the aortic arch curve was displaced to the right.5. The carotid sinus system had the greater gain (with reference to limb vascular resistance) and exhibited a greater maximal capacity to alter vascular resistance reflexly.6. When the carotid and aortic systems were activated simultaneously by distension, the reflex depressor responses were summed, essentially by a process of simple addition. A carotid-induced pressor response obscured a simultaneous aortic-induced depressor response of equal magnitude.7. In five dogs studied, the functional reflexogenic area of the aortic arch did not extend distally beyond the origin of the left subclavian artery.