The present study has been undertaken to clarify the ascending projections of the inferior colliculus (IC) to the posterior thalamus, particularly the medical geniculate body (MGB), using the Nauta-Gygax and Fink-Heimer methods. The ventrolateral part of the central nucleus of IC projects mainly to the laminated ventral principal part and the lateral portion of the magnocelluar part of MGB. The projections to the laminated ventral principal part are topically organized mediolaterally, i.e., the medial laminae of the central nucleus of IC project to the medial laminae of the ventral principal part of MGB, whereas the more lateral laminae of the central nucleus send fibers to the more lateral laminae of the ventral principal part of MGB. The projections of the central nucleus of IC to the laminated ventral principal part of MGB also appear to be organized anteroposteriorly and dorsoventrally. The dorsomedial part of the central nucleus of IC projects to the ventromedial portion of the ventral principal part, the magnocellular part, and the ventral portion of the dorsal principal part of MGB. The external nucleus of IC is situated lateral to the central nucleus and extends anteriorly to form the anterior extremity of IC. It projects to the magnocellular and ventral principal parts and to the ventral portion of the dorsal principal part of MGB, including the deep dorsal nucleus of Morest (1964). In addition, it sends fibers to the deeper layers of the superior colliculus (stratum griseum intermediale and profundum) and to the anterior pretectal and suprageniculate nuclei. The inferior colliculus projects to the nucleus of the brachium of the inferior colliculus and the interstitial nucleus of the inferior brachium and to the central gray of the midbrain. Each part of IC is connected with other parts by intrinsic fibers. IC sends fibers to the dorsal part of the contralateral inferior colliculus, particularly the dorsomedial part of the central nucleus, through the commissure of the inferior colliculus. Only a few fibers pass by way of the inferior brachium to MGB on the contralateral side.