Whole-cell currents were investigated in cultured rat retinal pigment epithelial (RPE) cells. Two voltage-dependent conductances were discriminated. First, at potentials more positive than -30 mV, a time-dependent outward current was activated. Inhibition by Ba2+ (10 mM) and 4-aminopyridine (10 mM) indicated that this current was carried by potassium ions. This current showed no inactivation during 5 sec depolarizations. Second, an inward current, sensitive to Ba2+ (10 mM) and 4-aminopyridine (10 mM), was activated at potentials more negative than -70 mV. Under extra- and intracellular potassium-free conditions, both currents disappeared. In summary, cultured rat RPE cells expressed one potassium conductance similar to the delayed rectifier and one similar to the inward rectifier. The delayed rectifier expressed characteristics comparable with those known in mammalian species and different from those in non-mammalian species.