The afferent projections to the inferior colliculus of the rat were studied using the method of retrograde transport of horseradish peroxidase (HRP). Following large injections of HRP into the central nucleus, cells within the cochlear nuclei, superior olivary complex and auditory cortex were stained. Within the contralateral dorsal cochlear nucleus, fusiform cells were heavily labeled. Giant cells were also labeled in deeper layers. In the contralateral ventral cochlear nucleus, virtually all major cell types were labeled, with some types being labeled in greater numbers than others. Octopus cells of posteroventral division of ventral cochlear nucleus (PVCN) were never labeled. HRP-positive cells were found in ipsilateral and contralateral lateral superior olivary nucleus (LSO), ipsilateral medial superior olivary nucleus (MSO), ipsilateral and contralateral lateral nucleus of the trapezoid body (LTB), ipsilateral ventral nucleus of the trapezoid body (VTB), and ipsilateral superior paraolivary nucleus (SPN). Pyramidal cells of layer V of auditory cortex were heavily labeled. Small injections of HRP into the central nucleus resulted in labeled cells within restricted regions of the cochlear nuclei, superior olivary complex and auditory cortex. Injections into dorsal regions of the central nucleus resulted in cells labeled in ventral regions of the dorsal and ventral cochlear nuclei, and in lateral regions of LSO. These regions contain neurons which are considered to have low best frequencies. Injections placed in more ventral regions of the central nucleus led to labeling of cells in more dorsal regions of the cochlear nuclei and more medial regions of LSO in agreement with the tonotopical progressions within these structures.