The rate-limiting step in Na transport by the renal cortical collecting duct is the apical amiloride-sensitive Na channel (ENaC), which is composed of three recently identified subunits (α, β, and γ). The immature CCD has a low rate of active Na transport and a very low Na-channel activity. To explore the mechanism, expression of the three ENaC subunit genes was studied in rat kidney at different developmental stages. Reverse transcription followed by polymerase chain reaction (rt-pcr) and agarose gel electrophoresis was used for specific mRNA detection. GAPDH mRNA, which did not show any age-related changes, was used as a positive control. All products were also partially sequenced to confirm their identity.