1. Whole-cell potassium currents of freshly isolated human (adult and fetal) and monkey (adult) retinal pigment epithelial (RPE) cells, as well as cultured human and monkey RPE cells were studied using the patch-clamp technique. 2. In freshly isolated adult cells of both species, two currents were observed in the voltage range from -150 to +50 mV: an outwardly rectifying current and an inwardly rectifying current. These currents were also found in cultured cells of both species. 3. The outwardly rectifying current in freshly isolated adult human and monkey cells and some cultured cells was evoked by depolarizing voltage pulses more positive that -30 mV. The current activated with a sigmoidal time course after a brief delay, and was virtually non-inactivating. The conductance associated with the current was half-maximal at -16.4 mV for fresh human cells and -13.5 mV for fresh monkey cells, but was shifted 16.0 and 17.7 mV in the positive direction in cultured human and monkey cells, respectively. The reversal potential of the current in both human and monkey cells matched the potassium equilibrium potential (EK) over a wide range of external potassium concentrations. This current was blocked by 20 mM tetraethylammonium. 4. A membrane current that exhibited inward rectification was observed with hyperpolarizing voltage pulses. The zero-current potential of this current was close to EK. This current was blocked by 2 mM Ba2+ and 2 mM Cs+. In cultured human and monkey cells, but not in fresh cells, this current exhibited an inactivation when voltage pulses were more negative than -120 mV. External Na+ was responsible for the inactivation, as the inactivation was removed in a Na(+)-free solution. 5. Membrane currents in freshly isolated fetal human RPE cells were remarkably different from those in adult cells. A transient outward current resembling the A-type potassium current was observed as the dominant membrane current in freshly isolated fetal human cells. This current activated when voltage pulses were more positive than -30 mV. It inactivated rapidly after reaching a maximal level. Application of 5 mM 4-aminopyridine (4-AP) completely blocked this current. Although this current was never observed in fresh adult cells, it was found in 33% of the cultured adult cells with similar kinetics, ion selectivity, and pharmacological properties. 6. In about 26% of the freshly isolated fetal human cells, a more slowly activating outward current, which resembled the delayed rectifier, was found to co-exist with the transient outward current.(ABSTRACT TRUNCATED AT 400 WORDS)