The effect of indacrinone (MK-196) on Cl^- transport through toad (Bufo bufo) skin epithelium was studied by the voltage clamping technique. At the transepithelial potential, V = 50 mV (serosal bath grounded) the unidirectional fluxes, governed by a Cl^- self-exchange diffusion pathway, were not affected by 1 mM racemic MK-196 in the outer bath. Likewise at V = o mV, the unidirectional fluxes as well as the active (net) inward flux of Cl^- were unaffected by MK-196. Voltage clamping the epithelium in the physiological range of potentials activated a Cl^- specific passive conductance that saturated for V ≤ -90 mV. The influx and efflux of Cl^- through this pathway were inhibited by MK-196, and the (passive)Cl^- current was inhibited in a dose-dependent way for [MK-196] ≥ 50 μM with about 70% inhibition for [MK-196] = 1 mM. The maximum Cl^- conductance was decreased without shifting the position along the V-axis of the inverted S-shaped conductance-voltage relationship. The time constants for the voltage-stimulated Cl^- conductance activation were not affected by MK-196 (50 μM ≤ [MK-196] ≤ 1 mM). The (+) and (-) isomers and racemic MK-196 affected the voltage-dependent Cl^- conductance in similar ways. It is concluded that MK-196 has the properties of a Cl^- channel blocker which is specific for the voltage-dependent Cl^- permeability of the epithelium. The time course for development of inhibition exhibited a fast (min) and a slow (h) component. The fast component may reflect a direct interaction of MK-196 with an extracellular site of the Cl^- channel, whereas the slow one may reflect impairment of a metabolic pathway regulating the Cl^- permeability, or an intertaction of MK-196 with a cytoplasmic site of the anion permeation pathway.