The distributions of mRNAs coding for three different isoforms of the alpha and two of the beta subunits of Na,K-ATPase were studied in the rat cochlea using in situ mRNA hybridization. The alpha1 and beta1 subunit isoforms were widely distributed and appear to mediate the Na and K transport needs of most cochlear cells. The mRNAs encoding the alpha2 and alpha3 subunit isoforms were present in glial and neuronal types, respectively. The only subunit isoform mRNAs found in the stria vascularis were alpha1 and beta2. This suggests that Na,K-ATPase consisting of the alpha1 and beta2 subunits plays a critical role in the maintenance of high extracellular K(+) levels in the cochlear endolymph and appears to transport K(+) ions against very low levels of intracellular potassium.