The retrograde transport of horseradish peroxidase (HRP) was used to locate cell bodies of afferent neurones with axons in the aortic nerves within the nodose ganglia of cats and rabbits. 2. In the rabbit HRP-labelled cells were localized to the medial aspect of the ganglion in the vicinity of the entry of the superior laryngeal and aortic nerves. 3. A similar grouping of labelled cells in rostro-medial aspects of the ganglion was seen in the cat, though there were many other labelled cells distributed throughout the ganglion. It is suggested that the former cells may be barosensory and the latter chemosensory. 4. Extracellular recordings were made from baroreceptor afferents in the nodose ganglia of rabbits and cats using glass micro-electrodes. Recordings sites were confined to rostral regions of the ganglia. 5. The projections of single baroreceptor afferents were studied by antidromic activation from the medulla oblongata using electrical stimulation applied through tungsten--glass micro-electrodes. Evidence is presented on the basis of depth--threshold contours and antidromic latency which indicates that stimulation of the main axons can be distinguished from stimulation in areas of branching and presumed termination. 6. In cats aortic baroreceptor afferents gave branches to either medial (three out of six) or lateral (three out of six) subnuclei of the nucleus tractus solitarius (n.t.s.). One neurone had additional projections to the contralateral medial subnuclei of the n.t.s. 7. In rabbits the afferents have branches mainly to lateral subnuclei of the n.t.s. (seven out of eight neurones) with only one giving branches to the medial subnucleus of the n.t.s. and additionally to the nucleus alaris. 8. Projections to the medulla extending beyond the immediate vicinity of the n.t.s. were found in either species.
