The distribution of Na +,K+-ATPase α1 subunit mRNA was identified using in situ hybridization in the developing rat cochlea. The expression of α1 subunit mRNA in stria vascularis (SV) was observed in all time points studied, 1 to 30 postnatal day (pnd) rats. The adult expression level was attained between 11 to 14 pnd. Surprisingly, α1 subunit mRNA in spiral ligament (SL) and spiral limbus (SLi) was expressed in a more distinct time-dependent manner. At 7 pnd, the α1 subunit mRNA expression was observed initially in the tissues of the SL. At 11 pnd, α1 subunit expression appeared in SLi. Between 11 and 14 pnd, an adult-like pattern of Na+,K+-ATPase α1 subunit mRNA expression was attained in the SL and SLi. These data suggest that the expression of Na+,K+-ATPase α1 subunit mRNA in these areas are closely related to the development of the rat EP, as its expression in the stria vascularis. © 1994.