An electron and light microscope study of the ventral division of the medial geniculate body using Golgi techniques, neurofibrillar stains and experimentally induced secondary degeneration. Geniculo-cortical relay cells and Golgi type II interneurons are easily recognized in the Golgi picture; under the electron microscope the two cell types and their dendrites can be identified on the basis of their different plasma structure. Synaptic arrangement of this region is analysed by a detailed comparison between the EM structure and the Golgi picture of the dendrites and axonal arborizations, the neurofibrillar picture of synapses, as well as degeneration pictures after lesions of the inferior colliculus and of the auditory cortical fields. Criteria derived from the light microscope study concerning size and distribution of various nerve endings, as well as secondary degeneration, have been used to identify various axon endings under the EM. The synapses of specific auditory afferents are confined to synaptic clusters arranged around the interdigitating dendritic tufts of relay cells. Although these synaptic clusters resemble in many respects the synaptic glomeruli of the lateral geniculate body, they cannot be termed as such due to lack of a glial capsule and also to less regularity in topographic arrangement of their various axonal and dendritic elements. It became nevertheless possible to identify in the synaptic clusters (1) the terminals of the specific auditory afferents of inferior collicular origin, (2) the axon terminals of Golgi II type neurons and, with less certainty (3) another axonal ending that might belong to descending cortical fibers originating from the auditory region. All types of axons have contacts with the dendrites of relay cells. It is not quite clear, whether the dendrites of Golgi type II cells are involved in these synaptic clusters, but this is very probable. Axo-axonic synapses between type (1) and (2) are frequent, type (1) (the auditory afferent) being always presynaptic. Very numerous synaptic contacts on the distal dendrites of both main cell types are found outside the synaptic clusters. They do not belong to either type (1) or type (2), many look like type (3) and may undergo degeneration after lesions of auditory cortical regions.