Single unit responses were recorded in the medial geniculate body (MGB) of anesthetized cats. In response to acoustical stimulation the properties of response latency, discharge pattern, frequency tuning, binaural interaction, and habituation were examined to allow an appraisal of the differentiation of the MGB by electrophysiological means. It is found that definite boundaries can be determined at which there is a distinct change in response properties; the position of these "physiological boundaries" seems to correspond with the boundaries between the seven subnuclei of the MGB described by Morest (Morest, D. K. (1964) J. Anat. 98: 611-630) in Golgi-stained material. Using these physiological boundaries to determine unit locations, population comparisons are made allowing the description of each subnucleus in terms of its auditory response properties. It is suggested that these properties, together with the limited information gained from Nissl cytoarchitecture, are sufficient to describe the location of physiological recording sites in the MGB.