Morphometric analyses of the cross-sectional areas were performed on spiral ganglion and vestibular ganglion cells in the CBA/CBA mouse from the 16th gestational day up to the age of 1 month. Vestibular ganglion cells undergo a period of rapid growth between the 4th and 12th postnatal day. Spiral ganglion cells revealed a rapid increase in cross-sectional area during the latter part of the 2nd postnatal week. Increases in cell volume could not be correlated to changes in individual morphological or electrophysiological events, but probably reflect adaptation to function. In 1-month-old animals, the cross-sectional area of both vestibular and spiral ganglion cells had decreased as compared with the 2nd postnatal week.