Neuronal immunoreactivity for the calcium binding proteins, parvalbumin and calbindin, has been examined immunohistochemically in the monkey medial geniculate complex. Neurons containing one or other of these proteins were present in most (but not all) regions of the complex and were differentially distributed; parvalbumin immunoreactive neurons tended to form densely packed clusters in the ventral nucleus and rather loose arrangements in the anterodorsal and posterodorsal nuclei, whereas most calbindin neurons were scattered, with concentrations in the caudal end of the posterodorsal and in the magnocellular nuclei. In fluorescent tracer experiments combined with immunohistochemistry, it was found that parvalbumin and calbindin neurons project to layer IV and to layer I, respectively, of the primary auditory cortex. These results suggest the presence of parallel and chemically differentiated geniculocortical pathways which make different contributions to the neuronal circuitry of the auditory cortex.