The postnatal expression of five Na, K-ATPase α (α1, α2, α3) and β (β1, β2) subunit isoforms in the rat cochlea was investigated by immunocytochemistry. High levels of expression of the α1 and β2 isoforms were observed in stria vascularis (SV) at all developmental stages. α1 and β1 isoforms showed a distinct time-dependent developmental expression pattern in tissues of the spiral ligament (SL) and spiral limbus (SLi). Limited, temporary expressions of α2 and α3 subunit isoforms were found in SV and SL. Expression of each isoform was also seen in organ of Corti (OC), spiral ganglion (SG), cochlear nerve (CN) and Kolliker's Organ (KO). These observations suggest that individual isoforms may exert specific actions postnatally during final cochlear maturation © 1995 Informa UK Ltd All rights reserved: reproduction in whole or part not permitted.