WE examined the effect of neonatal sensorineural hearing loss on synaptic density in the central nucleus of the inferior colliculus (ICC) of adult cats to evaluate the role of auditory experience in synaptogenesis. Three groups of animals were used: bilaterally deafened, uni-laterally deafened and normal hearing controls. Synaptic density in bilaterally deafened animals was significantly lower than in normal hearing animals. By contrast, there was no significant difference in synaptic density between normal hearing animals and unilaterally deaf animals. These results demonstrate, for the first time, that a sensorineural hearing loss during development can affect synaptogenesis in the auditory midbrain.