Various epithelia are exposed to shear forces but little is known about the impact of these forces on epithelial ion channels. We investigated the effect of laminar shear stress on epithelial sodium channels (ENaCs), derived from rat colon (rENaC) and Xenopus kidney cells (xENaC), using the Xenopus oocyte expression system. In whole-cell recordings, activation of a fluid stream to induce shear forces in physiologically relevant ranges, activated amiloride-sensitive ENaC currents. Preceding pharmacological activation of ENaC with channel openers (Gd3+ and glibenclamide for xENaC; Zn2+ for rENaC) largely abolished shear stress induced currents. In contrast, proteolytic cleavage with trypsin, potentiated the effects. Using excised patches in the outside-out configuration, we were able to analyse shear stress effects at the single channel level. We found, that shear stress directly increased the ion channel open probability (Po) without affecting the number of active channels in the excised membrane patch (N). The increased Po was either due to a significantly increased mean open time (rENaC) or a decreased mean closed time (xENaC). In addition, there were no changes detectable in ion selectivity (Li+ > Na+ ). Taken together, this data suggest, that laminar shear stress leads to an increase in the activity of membrane located ENaCs rather than recruiting new channels from intracellular stores. Supported by DFG, FR2124