We have analysed the postnatal volume increase of the cochlear nucleus subdivisions from birth to senescence in the Mongolian gerbil. All subdivisions showed the most prominent growth in the 2nd week of life before the onset of hearing on day 12. After the onset of hearing, the dorsal and the postero-ventral portion of the cochlear nucleus showed only a moderate degree of further growth. The antero-ventral cochlear nucleus, however, almost doubled in volume after the onset of hearing, reaching a maximum in 4-month-old animals. In ageing gerbils, we observed a small but systematic reduction of the cochlear nucleus volume. To further assess which elements contribute to the growth of the antero-ventral cochlear nucleus after the onset of hearing, we analysed the somal size of the neurones. Before the onset of hearing, somal and volume growth were highly correlated. However, somata had reached an adult size by the onset of hearing. We conclude that the growth of the antero-ventral cochlear nucleus after the onset of hearing is due to changes in the neuropil.