The purpose of the present study was to determine whether the renal inner medulla expresses mRNA for the rat epithelial Na⁺ channel (rENaC) and, if so, to define its regulatory properties using a low-Na⁺ diet model. We detected α, β and γ subunit mRNA in rat renal inner medulla using reverse transcriptase-polymerase chain reaction (RT-PCR) with primers specific for rENaC α, β and γ subunits. Moreover, we have developed a specific probe for the α subunit using RT-PCR with rENaC α-subunit-specific primers. The resulting cDNA was verified by sequencing and was then used in Northern blot analysis of distal colon, whole kidney and inner medulla. The probe for the rENaC α subunit hybridized not only to distal colon RNA but also to inner medulla RNA derived from rats fed a normal diet. Furthermore, we examined the effect of a low-Na⁺ diet on α, β and γ subunit mRNA expression of rENaC using full-length cDNA as a probe. A marked elevation of rENaC α subunit mRNA abundance in the inner medulla was observed in response to a high plasma aldosterone concentration induced by dietary Na⁺ deprivation. On the other hand, neither β nor γ subunit mRNA expression was enhanced by a low-Na⁺ diet. From these results, it is suggested that rENaC is responsible for Na⁺ transport in the renal inner medulla and that is probably regulated via transcriptional control of the α subunit of ENaC.