These studies have enabled the first characterization of the properties of ion channels in adult oligodendrocytes. Cell-attached recordings from cultured adult bovine cells showed channel activity with 140 mM KC1 in the patch pipette; the amplitude of the currents was increased with increasing membrane hyperpolarization. This channel, with a conductance of 29 pS, was selective for inward K' current; little or no outward current was measured for large depolarizing voltage steps. The channel open time was strongly dependent upon membrane potential, with membrane hyperpolarization decreasing the mean open time 100-fold over a range of 80 mV; at the resting potential of the cell the mean open time of the channel was in excess of 50 ms. Decreasing the Concentration of K' in the pipette diminished the channel conductance with no significant effect to alter the channel open time dependence on potential. The rectification and kinetic properties of the channel would be consistent with a physiological role for the channel in the regulation of external Kt near active neurons; in particular the effect of membrane depolarization to cause maintained channel open duration could be important when the driving force for inward potassium movement through oligodendrocyte membrane was low. Channels selective for outward potassium movement were seen with inside-out excised patch recordings with symmetrical potassium concentrations across the patch; the density of these channels in the bovine membrane was low.