Despite large changes in water and salt intake, the kidney is able to maintain the extracellular osmolarity and volume within narrow margins (Verrey et al., 2000). Such fine control requires specific factors or hormones; among them, aldosterone and antidiuretic hormone (ADH)(vasopressin in mammals) play the key role. The main effector for the sodium balance is the epithelial sodium channel expressed in the apical membrane of the principal cell located in the distal nephron (distal convoluted tubule and collecting duct). The effect of aldosterone on sodium transport has been highly conserved throughout the evolution of vertebrates, whereas that of vasopressin varies from species to species. In the urinary bladder of the toad (Girardet et al., 1986) and in the cortical collecting duct (CCD) of the rat (Reif et al., 1986), ADH increases sodium transport through a cAMP-dependent mechanism, via rapid (within minutes) nongenomic activation of ENaC and, later (within hours), by genomic activation (or repression) of a number of genes which have been identified recently (Robert-Nicoud et al., 2001). The natriferic effects of cAMP and aldosterone are synergistic (Girardet et al., 1986). The molecular mechanism by which cAMP modulates ENaC activity to rapidly increase net sodium reabsorption remains a matter of controversies. Setting aside an increase in the unit conductance of ENaC, which has never been observed, one possible mechanism is that cAMP mediates an increase in open probability (Po) with no change in the number of molecules expressed in the apical membrane of the principal cell. An alternative possibility is that the hormonal effect is due to an increased insertion of new channels at the cell membrane and/or a decreased retrieval from the surface, leading to an increased number (N) of channels at the plasma membrane. The precise quantification of ENaC protein (N) present in the membrane and the measurement of its Po have been difficult for two main reasons: first, the overall density of ENaC molecules in the apical membrane of principal cells is very low, maybe 30–50 channels per cell. Under a normal salt diet and water repletion, ENaC protein is undetectable in the apical membrane of the CCD principal cell by classical immunohistochemical technique (Masilamani et al., 1999; Loffing et al., 2000) or by patch clamp (Pacha et al., 1993). This does not mean, however, that active ENaCs are not present in the apical membrane, but this small pool may be just undetectable with the available methodology. The immunohistochemical and physiological localization of ENaC at the apical membrane is seen only when the animal is put under low salt diet for a couple of days or weeks. Long term salt restriction (Pacha et al., 1993; Masilamani et al., 1999; Loffing et al., 2000) or short-term (15 h) Na deprivation (Frindt et al., 2001) lead to a relative hypovolemia, providing the physiological stimulus for aldosterone secretion by the adrenals. Second, ENaC displays some unique gating properties, as assessed by patch clamp experiments. Hormoneinduced changes in Po could theoretically be detected by patch clamp as for other ionic channels. But, as discussed by Palmer and Garty (2000), the open probability (Po) is highly variable; even two channels in the same patch may have widely different Po values. ENaC may exist in different gating modes. Therefore, ENaC could switch from one mode to another under the influence of a variety of factors or hormones. In addition, changes in opening and closing rates could account for the great variability of the observed Po (Palmer and Garty, 2000). Firsov et al.(1996) reported a quantitative and sensitive method …