In the present study we have analyzed the membrane currents of mature oligodendrocytes in cultures from dissociated fetal mouse cerebral hemispheres and explant cultures from fetal mouse spinal cord. Both types of oligodendrocytes showed large voltage-dependent, but time-independent inward and outward currents that were partially blocked by Ba2+. In addition, time- and voltage-dependent inward and outward currents were observed in a minority of oligodendrocytes from spinal cord. All voltage-dependent currents were completely blocked by Ba2+, and inward currents were completely blocked by Cs+, suggesting that they are mediated by K+ channels. Current-voltage curves of mouse spinal cord oligodendrocytes varied from being linear to outwardly or inwardly rectifying. In contrast, oligodendrocytes cultured from mouse brain always showed an inward rectification of the current voltage relation and a lack of time-dependent currents. It thus appears that mature oligodendrocytes in explant cultures of mouse spinal cord, in contrast to oligodendrocytes from dissociated brain, consist of different cell populations that are distinguished by their expression or active state of K+ channels.