The active estrogen, 17β-estradiol (E2), exerts a gender-specific anti-secretory effect in distal colon (1). This could provide an explanation for the fluid retention observed in females during periods of high circulating plasma E2. However, an effect of E2 on ion transport in other organs contributing to electrolyte and fluid homeostasis is unknown. The aim of this study was to determine whether E2 has a chronic effect on Na+ absorption in the renal cortical collecting duct using the M1 kidney cortical collecting duct cell line and to reveal the molecular mechanisms underlying effects of E2 on the amiloride-sensitive epithelial Na+ channel ENaC. Electrophysiological experiments using the Ussing chamber technique, demonstrated that treatment with E2 (25 nM) for 24 hrs increased the amiloride-sensitive short-circuit current (ISC) in M1 cells (Control 1.4±0.2 μA/cm2, E2 3.1±0.6 μA/cm2; n=7). This increase in Na+ current was not attributable to changes in the ouabain-sensitive Na+/K+-ATPase pump current (Control 11.3±1.6 μA/cm2, E2 8.9±1.5 μA/cm2; n=4). Moreover, there were no changes in the abundance of the α- or β-Na+/K+ -ATPase subunits. The increase in the amplitude of the Na+ current could be explained by changes in the expression of the Na+ channel ENaC subunits. To test this hypothesis, we looked at changes in the abundance of α-, β-, and γ-ENaC subunits using Western-blotting. We found that the treatment with E2 for 24 hrs did not produce any detectable change in the abundance of the α- or β-ENaC subunits. However, E2 induced a 2-fold increase in the abundance of the γ-ENaC subunit (2.0±0.1, n=4). In conclusion, E2 chronically increases the ENaC mediated absorption of Na+ in CCD cells. The results suggest that the pro-absorptive effect of E2 could be explained by an up-regulation of the γ-ENaC subunit expression in the apical cell membrane.