Abstract
1. In anaesthetized dogs lesions of 4 mm diameter centred on the obex were placed in the medulla dorsal to the hypoglossal nucleus. 2. Placing of the lesions almost totally abolished the reflex responses to distension of the pulmonary vein-left atrial junctions and to low intensity stimulation of the aortic nerv. The reflex response to high intensity stimulation of the aortic nerve was reduced and the reflex response to carotid artery occlusion remained unaltered. 3. Mid-collicular decerebration did not affect the results. 4. The results are consistent with a hypothesis that afferents from the carotid sinus baroreceptors synapse in the nucleus of the tractus solitarius rostral to the obex, whereas afferent fibres from the aortic baroreceptors and atrial receptors synapse in the intermediate portion of the nucleus of the tractus solitarius close to the obex.