17β-estradiol (E2), regulates Cl- secretion in distal colon in a gender-specific manner via inhibition of the basolateral K+ channel KCNQ1 (1). The fluid retention observed in females during periods of high circulating plasma E2 could be explained, in part, by the anti-secretory effect of E2 observed in the distal colon of female rats (1). The physiological role of fluid conservation in high E2 states may be to facilitate uterine expansion and restructuring in advance of embryo implantation and also to positively impact upon cardiovascular dynamics. The effect of E2 on ion transport in other organs contributing to electrolyte and fluid homeostasis in females has not been characterised. For example, E2 modulation of Na+ re-absorption in the renal cortical collecting duct could contribute to fluid conservation observed during periods of high circulating plasma E2. The aim of this study was to determine whether E2 had an effect on Na+ re-absorption in the renal cortical collecting duct using the well-characterised M1 mouse renal cortical collecting duct line (M1 cells) and to reveal the molecular mechanisms underlying effects of E2 on ENaC. Treatment with E2 (25 nM) for 15 min increased the amplitude of the amiloride-sensitive short-circuit current (ISC) in M1 cells grown as polarized monolayers in Ussing chambers (Control 1.5±0.4 μA/cm2, E2 2.8±0.4 μA/cm2; n=6). Experiments in amphotericin-B perforated epithelia, using ouabain, demonstrated that this response was not attributable to an increase in the Na/K -ATPase activity (Control 2.9±0.2 μA/cm2, E2 3.3±0.8 μA/cm2; n=4). Moreover, there was no change either in total or basolateral surface abundance of the α-Na/K -ATPase subunit. An explanation for the increase in the amplitude of the amiloride-sensitive ISC observed above could come from changes in the total or apical surface expression of the ENaC subunits. Then, we looked at changes in the abundance of α-, β-, and γ-ENaC subunits using biotinylation, Western-blot and immunofluorescence. After 30 min treatment with E2 25 nM, there was not detectable change in either the total or apical surface expression of the α- and β-ENaC subunits. However, there was a significant increase in the apical membrane abundance and localization of the γ-ENaC subunit (20±2% compared with the control, n=4). In conclusion, this work demonstrates that E2 can rapidly increase the amplitude of the amiloride sensitive ISC in M1 cells. The results presented above rule out the involvement of the Na/K -ATPase in the process. However, E2 can induce an increase in the apical surface abundance of the γ-ENaC subunit, which could provide an explanation for the pro-absorptive effect observed for E2 in M1 cells.