Deafness leads to different degrees of changes along the auditory pathway depending on the timing and duration. Here we investigate the effect of unilateral and bilateral neonatal deafness (ND) compared to bilateral adult deafness (AD) on the volume development of the cochlear nucleus (CN) of rats.Unilateral ND was induced in rats on postnatal (P) day 10 by intracochlear injection of neomycin. Bilateral ND was induced by intraperitoneal injection of kanamycin (P10-20). Bilateral AD was induced by injection of ethacrynic acid and kanamycin in 3- and 12-months old rats. Hearing thresholds of >90 dB verified successful deafening. One month after deafness, rats were perfused with 4% paraformaldehyde and the brain was dissected. Subsequently, brain sections were prepared, the CN examined under a light microscope, photographed and analyzed. The length, width, area, and volume of the CN were determined.CNs of neonatally deafened rats showed significantly smaller volumes. Compared to normal hearing rats, bilateral deafness resulted in a 31.41% smaller CN volume, while unilateral deafness resulted in a 36.93% smaller CN volume on the deaf side. In both cases, the ventral part was most affected by the volume reduction. In young and old AD rats, the CN volume was reduced by 10.43% and 1.37%, respectively, and did not reach significance.In contrast to AD, ND results in severe anatomical changes in the central auditory system. This confirms the importance of diagnosing deafness as early as possible to prevent reduced or impaired development of the immature auditory system using hearing prostheses.