Several Cl(-) channels have been described in the native renal tubule, but their correspondence with ClC-K1 and ClC-K2 channels (orthologs of human ClC-Ka and ClC-Kb), which play a major role in transcellular Cl(-) absorption in the kidney, has yet to be established. This is partly because investigation of heterologous expression has involved rat or human ClC-K models, whereas characterization of the native renal tubule has been done in mice. Here, we investigate the electrophysiological properties of mouse ClC-K1 channels heterologously expressed in Xenopus laevis oocytes and in HEK293 cells with or without their accessory Barttin subunit. Current amplitudes and plasma membrane insertion of mouse ClC-K1 were enhanced by Barttin. External basic pH or elevated calcium stimulated currents followed the anion permeability sequence Cl(-)>Br(-)>NO3(-)>I(-). Single-channel recordings revealed a unit conductance of ~40pS. Channel activity in cell-attached patches increased with membrane depolarization (voltage for half-maximal activation: ~-65mV). Insertion of the V166E mutation, which introduces a glutamate in mouse ClC-K1, which is crucial for channel gating, reduced the unit conductance to ~20pS. This mutation shifted the depolarizing voltage for half-maximal channel activation to ~+25mV. The unit conductance and voltage dependence of wild-type and V166E ClC-K1 were not affected by Barttin. Owing to their strikingly similar properties, we propose that the ClC-K1/Barttin complex is the molecular substrate of a chloride channel previously detected in the mouse thick ascending limb (Paulais et al., J Membr. Biol, 1990, 113:253-260).