In mammals nine genes code for Cl transporting CLC proteins. Several CLCs are involved in human genetic diseases. Triggered by the finding of Accardi & Miller (Nature 427:803) that the bacterial CLC-ec1 is a secondary active, highly electrogenic, Cl-proton exchanger we investigated specifically CLC-5, that is mutated in Dent's disease. We found that the outwardly rectifying current carried by CLC-5 is associated with a significant movement of protons. Detailed measurements allowed us to conclude that CLC-5 and CLC-4 function as Cl-proton antiporters, while other CLC's (CLC-0, CLC-2, CLC-Ka) do not transport protons to a significant amount and are thus passive ion channels. Analysis of the CLC-0 mutation E166D, in which the "gating glutamate" is substituted by Asp shed further light on the relationship between the channel and the transporter character of CLC proteins. In parallel studies we investigated pharmacological properties of renal CLC-K/barttin channels. We found that niflumic acid (NFA) increases CLC-Ka/Kb currents in the 10–1000 mM range. Flufenamic acid derivatives or high doses of NFA inhibited CLC-Ka, but not CLC-Kb, or blocker insensitive CLC-Ka mutants, indicating that the activating binding site is distinct from the blocker site. These molecules provide a starting point for identification of diuretics or drugs useful in the treatment of Bartter's syndrome.