The cytoarchitecture, myeloarchitecture, and neocortical connectivity of the rat medial geniculate body (MGB) were comprehensively studied in adult and immature rats to define major anatomical divisions and nuclei. The MGB is a highly intricate structure composed of the ventral (MGv), dorsal (MGd), and medial (MGm) divisions and component nuclei, each having reciprocal connections with auditory neocortex. The MGv lies inferior to the midgeniculate bundle and extends to the rostral, but not caudal MGB tip. The MGv is composed of ventral and ovoid nuclei bounded by a marginal zone, each region containing dark staining small and medium sized, densely packed neurons shown to have tufted dendritic morphology; in contrast to the MGd, but similar to the dorsal lateral geniculate nucleus, only the perikarya of MGv neurons stain for Nissl in early postnatal material. Ventral nucleus cells align with afferent brachial axons, which penetrate the nucleus in a dorsoventral direction, whereas rostrocaudal cellular arrays are retrogradely labeled after injections of horseradish peroxidase (HRP) into auditory cortex. The ovoid nucleus is a double spiraled structure encircled and penetrated by afferent fibers that determine the orientation of constituent perikarya. Neurons in the transition zone align with a spray of axons emanating from the juncture of the ovoid and midgeniculate bundles. Marginal zone neurons are oriented in parallel to the free geniculate wall. The MGd resides within and superior to the midgeniculate bundle, and is composed of several nuclei that stain palely for myelin. In immature material, both dendritic processes and somata in the MGd stain for Nissl with our protocol; many of these cells show a stellate arborization pattern that distinguishes this region from the MGv, but is similar to the staining pattern of immature neurons of the lateral posterior nucleus. The adult dorsal nucleus has medium-sized, loosely packed neurons. The deep dorsal nucleus is situated among the fibers of the midgeniculate bundle and contains loosely packed round and fusiform cells; the latter cell type constitutes a minor proportion of the adult neuronal population but the major cell type in immature animals. The caudodorsal nucleus, which occupies the caudal tip of the MGB and rostrally courses superior to the dorsal nucleus, contains small, dark staining multipolar cells; the ventrolateral nucleus courses inferior to the MGv. The suprageniculate and limitans nuclei are included in the auditory thalamus on the basis of connections with auditory neocortex; the former has medium to dark staining mixed-sized cells, and the latter has densely packed cells which form a vertical column.(ABSTRACT TRUNCATED AT 400 WORDS)