We evaluated the consequences of neonatal cochlear destruction upon ascending projections to the inferior colliculi. Unilateral cochlear ablations were performed in both neonatal and adult gerbils. Four to 12 months later, the inferior colliculus (IC) was examined physiologically and injected unilaterally with horseradish peroxidase (HRP). The number of labeled cells was determined bilaterally in all three divisions of cochlear nucleus (CN) and in the medial superior olive (MSO). In both experimental groups, transneuronal changes within the deafferented CN were greater in the ventral divisions than in the dorsal division. On the unoperated side the magnitude of projections from CN to the inferior colliculi was altered in animals lesioned as neonates. Following HRP injections into the IC on the unoperated side, the number of ipsilaterally labeled cells in CN (unoperated side) was significantly greater in the neonatal experimental group than in adult experimental and control animals. These anatomical changes were accompanied by increased ipsilaterally evoked excitatory activity recorded in the IC on the unoperated side. Following HRP injections into the IC on the ablated side, the number of contralaterally labeled cells in CN (unoperated side) was significantly reduced in animals lesioned as neonates as compared with control animals. The number of labeled cells in ipsilateral MSO was not significantly different across groups. Our interpretation is that unilateral cochlear ablation in neonatal gerbils results in an increase in the magnitude of ipsilateral projections and a decrease in the magnitude of contralateral projections from CN on the unoperated side to the inferior colliculi. These data suggest that the normal pattern of innervation of the IC results, in part, from interactions among afferent projections.