The neurones of the afferent and efferent auditory pathways were labelled by axonally transported horseradish peroxidase. The first neurone of the ascending system is the acoustic nerve with the spiral ganglion. These fibres terminate mainly in the ventral cochlear nucleus (VCN). The neurones of the latter chiefly run to the contralateral superior olivary complex (SOC), whereas the neurones of the dorsal cochlear nucleus (DCN) terminate mainly in the central nucleus of the contralateral inferior colliculus (IC). The nerve cells of the superior olivary complex and nuclei of the lateral lemniscus (DLL, VLL) project to the IC. The IC neurones run mainly uncrossed to the medial geniculate body (MGB), i.e. the neurones of the central nucleus of the inferior colliculus terminate in the ventral nucleus of the medial geniculate body. This latter nucleus projects to the primary region A I of the auditory cortex, whereas the neurones in the pericentral medial geniculate body terminate in the associated auditory regions A II, EP and SF. The auditory areas of both hemispheres is well interconnected by commissural fibres. In addition to this ascending pathway there is a descending system. The parent cells correspond to pyramidal neurones in layer V in the auditory cortex. These neurones terminate in the medial geniculate body and in the pericentral nuclei of the inferior colliculus. From this, efferent neurones project to the DCN and SOC. The neurones of the olivocochlear bundles originate in the SOC; these bundles terminate predominantly on the surface of the outer hair cells in the organ of Corti.