T he terminal nephron segment, the inner medullary collecting duct (IMCD), absorbs Na+ by an electrogenic process that involves the entry through an apical (luminal) membrane Na+ channel. To understand the nature of this Na+ channel, we employed the patch clamp technique on the apical membrane of primary cultures of rat IMCD cells grown on permeable supports. We found that all ion channels detected in the cell-attached configuration were highly selective for Na+ (Li+) over K+. The open/closed transitions showed slow kinetics, had a slope conductance of 6-11 pS, and were sensitive to amiloride and benzamil. Nonselective cation channels with a higher conductance (25-30 pS), known to be present in IMCD cells, were not detected in the cell-attached configuration, but were readily detected in excised patches. The highly selective channels had properties similar to the recently described rat epithelial Na+ channel complex, rENaC. We therefore asked whether rENaC mRNA was present in the IMCD. We detected mRNA for all three rENaC subunits in rat renal papilla and also in primary cultures of the IMCD. Either glucocorticoid hormone or mineralocorticoid hormone increased the amount of alpha-rENaC subunit mRNA but had no effect on the mRNA level of the beta-rENaC or gamma-rENaC subunits. From these data, taken in the context of other studies on the characteristics of Na+ selective channels and the distribution of rENaC mRNA, we conclude that steroid stimulated Na+ absorption by the IMCD is mediated primarily by Na+ channels having properties of the rENaC subunit complex.