The mouse auditory neurons, hair cells and their supporting cells in the cochlea are considered to be generated mainly in the embryonic days and to be sustained throughout the whole life. In the present study, however, we observed that auditory ganglion cells in the spiral ganglia undergo apoptosis and mitosis in the suckling mouse (1- to 2-week-old C3H/HeJ mice) with a normal auditory system. In spiral ganglia at postnatal days 7 (P7) and 10 (P10), TUNEL (TdT-mediated dUTP nick-end labeling)-positive and morphologically apoptotic ganglion cells were found. Furthermore, by bromodeoxyuridine labeling, mitosis of auditory ganglion cells was found at P10 to P14. In a functional study of auditory brainstem response, we demonstrated that the C3H/HeJ mouse acquires the ability to hear airborne sound at P12 and this is the same time as the opening of their external acoustic meatus (EAM). These results indicate that C3H/HeJ auditory ganglion cells have the ability to proliferate even after opening of the EAM and the initial input of airborne sound. We found that postnatal apoptosis and mitosis after P7 also occurred in the greater epithelial ridge (GER) which is an important organ for maturation of the organ of Corti and is located around the inner hair cells. This indicates that GER cells are not only degenerated but also regenerated until their disappearance around P12. This is the first report on mammals to demonstrate that neuronal mitosis of spiral ganglion cells and that of GER cells occur not only in embryonic and neonatal development but also in postnatal development of the normal auditory system.