Cultured oligodendrocyte progenitor cells derived from the developing central nervous system (CNS) express a pattern of ion channels that is distinct from mature oligodendrocytes and other cell types of the CNS. In the present study, we used the whole-cell patch-clamp technique and the fura-2-based Ca++ imaging system to study the ion channel expression of oligodendrocyte progenitor cells derived from the optic nerves of adult rats. We found that the adult oligodendrocyte progenitor cell membrane is dominated by K+ currents, both delayed outward and inward rectifying. The inwardly rectifying K+ currents were often as large as the outward delayed rectifying K+ currents. The delayed rectifying outward currents were partially blocked by 50 mM tetraethylammonium or 1 mM 4-aminopyridine, but not by 2 or 5 mM BaCl2. This suggests that the delayed rectifier channels expressed by adult progenitor cells are different from those expressed by perinatal cells. Most adult oligodendrocyte progenitor cells showed no or only small A-type K+ currents. Both Ca++ and Na+ channels were also detected in these cells. Furthermore, adult progenitor cells responded to the neurotransmitters GABA and kainate and the pharmacology of these responses indicated that these cells express GABAA receptors and kainate receptors that are Ca++ -permeable. Our study suggests that adult oligodendrocyte progenitor cells are electrophysiologically distinct and that these cells share electrophysiological characteristics with both perinatal progenitor cells and immature oligodendrocytes. © 1995 Wiley-Liss, Inc.