In many epithelial tissues the amiloride-sensitive epithelial sodium channel (ENaC) is the rate-limiting step in sodium uptake. Recent studies reported that trypsin, a serine protease, leads to an increase in ENaC current by activation of the channel. For our experiments we expressed rat-ENaC in Xenopus laevis oocytes. We began our electrophysiological measurements 24 h after injection of the ENaC mRNA. With special hard- and sofware we were able to measure current (I_m), conductance (G_m) and capacitance (C_m) simultaneously. C_m is a measure of membrane surface area so that changes in C_m are a hint for endo- or exocytotic processes. We found a twofold increase in I_m and G_m after treatment with trypsin. Furthermore we could observe an increase in C_m of about 9 %. This increase in all three parameters implies functional insertion of preformed ENaC from intracellular pools into the plasma membrane. In another set of experiment we additionally added H8, a PKA blocker, to prevent PKA-dependent exocytosis. We could recognize an obvious decline of all parameters in presence of H8. Exocytosis inhibition by H8 is a hint for the presence of a putative trypsin-receptor in the cell-surface that mediates membrane insertion. From this data we conclude that ENaC is not only activated by trypsin, but also leads to an increase in membrane surface, which indicates insertion of preformed ENaC channels.