Currents in freshly dissociated adult human retinal pigment epithelial (RPE) cells were studied using the perforated patch-clamp technique. The zero-current potential (V0) averaged -48.9 +/- 7.7 mV (n = 50). Depolarizing voltage pulses from -70 mV evoked an outward current that activated with first-order kinetics and that did not inactivate during prolonged depolarizations. Repolarizing the membrane potential produced tail currents that reversed near the K+ equilibrium potential, indicating that the sustained outward current was carried mainly by K+. The outwardly rectifying K+ conductance (gK) had an activation threshold voltage near -60 mV and was half-maximal at -37 mV. Approximately 25% of gK was active at the average V0. The K+ current was nearly completely blocked by 2 mM Ba2+ but was relatively insensitive to 20 mM tetraethylammonium. The kinetics, voltage dependence, and blocker sensitivity of this current clearly distinguish it from delayed rectifier K+ currents previously identified in RPE cells. We conclude that the sustained outward K+ current may help establish the resting potential of the apical and/or basolateral membranes and may also participate in K+ transport across the RPE.