1. The third somatosensory area (SIII) was identified in the cat cerebral cortex by the recording of surface potentials evoked by deflection of a single contralateral mystacial vibrissa. A small amount of tritiated leucine was then injected at the center of the focus of evoked activity and, after a suitable survival period, the brain was prepared for autoradiography. 2. As defined by the presence of an autoradiographic injection, the SIII focus lay in a cytoarchitectonic field characterized in particular by the presence of very large pyramidal cells in layer V and corresponding to area 5a of Hassler and Muhs-Clement (24). 3. The terminal ramifications of corticothalamic cells, as outlined by axoplasmically transported label, formed clustered aggregations in the medial division of the posterior group of thalamic nuclei (Pom) and not in the ventrobasal complex (VB). This part of Pom is known to receive fibers from the spinal cord. 4. Injections of horseradish peroxidase primarily affecting area 5a retrogradely labeled cells in Pom but not in VB. 5. Injections of isotope in the two other foci of vibrissa-evoked activity usually recorded in each brain were invariably found to label a part of area 3b of the first somatosensory area (SI) in the case of the more anterior focus. The second focus sometimes lay in area 2 of SI and sometimes in the second somatosensory area (SII).