Immunohistochemistry was used to investigate the presence of Na,K-ATPase α- and β-subunits isoforms (α1, α2, α3, β1 and β2) in the cochlea of the mouse at different ages between embryological day (E) 19 and postnatal day (P)+30. α1 was mainly found in the stria vascularis and in the spiral ligament; it increased steadily from p+4. These data correlates well with the morphological and electrophysiological maturation of the cochlea. α3 predominated in the spiral ganglia and the cochlear nerve. This finding is well in accordance with reports that α3 seems to be associated with the nervous system. The β-subunit was found mainly in those tissues where staining of the α-subunit also was seen. Both subunits were localized in tissue regions where fluid regulation is expected to play an important role. For some isoforms, the expression pattern of Na,K-ATpase during development in the mouse is different from that in the rat. The expression of Na,K-ATPase and that of glucocorticoid receptors during development in the inner ear of the mouse show a similar pattern, which may indicate that glucocorticoid receptors could be involved in regulating the expression of Na,K-ATPase.