Liddle's disease is an autosomal dominant form of human hypertension resulting from a basal activation of amiloride-sensitive Na+ channels (ENaC). This channel activation is produced by mutations in the β- and/or γ-carboxy-terminal cytoplasmic tails, in many cases causing a truncation of the last 45-76 amino acids. In this study, we tested two hypotheses; first, β- and γ-ENaC C-terminal truncation mutants (β(ΔC) and γ(ΔC)), in combination with the wild-type α-ENaC subunit, reproduce the Liddle's phenotype at the single channel level, i.e., an increase in open probability (P(o)), and second, these C-terminal regions of β- and γ-ENaC act as intrinsic blockers of this channel. Our results indicate that αβ(ΔC)γ(ΔC)-rENaC, incorporated into planar lipid bilayers, has a significantly higher single channel P(o) compared to the wild-type channel (0.85 vs 0.60, respectively), and that 30-mer synthetic peptides corresponding to the C-terminal region of either β- or γ-ENaC block the basal-activated channel in a concentration-dependent fashion. Moreover, there was a synergy between the peptides for channel inhibition when added together. We conclude that the increase in macroscopic Na+ reabsorption that occurs in Liddle's disease is at least in part due to an increase in single channel P(o) and that the cytoplasmic tails of the β- and γ-ENaC subunits are important in the modulation of ENaC activity.