Earlier studies have shown that the efferent ductules (ED) of the male mouse are a target for estrogen. The loss of estrogen receptor (ER) function through either knockout technology (αERKO mouse) or chemical interference (pure antagonist, ICI 182 780) results in a failure of a major function of the ED, the reabsorption of testicular fluids. The purpose of this study was to test the hypothesis that estrogen controls fluid (water) reabsorption in the ED by modulating ion transporters important for passive water movement through a leaky epithelium such as the ED. Northern blot analysis was used to detect the mRNA levels for key ion transporters in the following experimental groups: 1) wild-type (WT) control for the 14-day experiment, 2) ERα knockout (αERKO) control for the 14-day experiment, 3) WT treated with ICI 182 780 (ICI) for 14 days, 4) αERKO treated with ICI for 14 days, 5) WT control for the 35-day experiment, and 6) WT treated with ICI for 35 days. Estrogen differentially modulated the mRNA levels of key ion transporters. ERα mediated carbonic anhydrase II mRNA abundance, and there was a decrease in Na /H exchanger 3 mRNA levels in the αERKO that appeared to be a cellular effect and not a direct estrogen effect. The loss of ERα control resulted in an increase in mRNA abundance for the catalytic subunit of Na -K ATPase α1, whereas an increase in the mRNA abundance of the Cl⁻/HCO₃⁻ exchanger and the chloride channel cystic fibrosis transmembrane regulator was significantly ERβ mediated. Our results indicate for the first time that estrogen acting directly and indirectly through both ERα and ERβ probably modulates fluid reabsorption in the adult mouse ED by regulating the expression of ion transporters involved in the movement of Na and Cl⁻.