The patch clamp technique was used to analyze single channel currents in intact and excised patches of glial cell membranes grown in primary cultures from newborn rat brain. Glial cells were morphologically identified by immunohistochemical staining for glial fibrillary acidic protein. Outward currents due to single channels were observed in recordings from both intact and excised patches obtained from the cell body region. The channel responsible for these currents was preferentially permeable to K+ because the reversal potential for this current was correlated with changes in the potassium equilibrium potential, when experimentally altered. The single channel conductance was 25 pS when measured between -20 and +20 mV in solutions with physiological K+ concentrations (10 degrees C). Channel gating was dependent on both the internal Ca2+ concentration and the membrane potential. Either depolarization of the membrane patch, or the addition of increasing Ca2+ concentrations to the internal surface, increased the probability of channel opening. Tetraethylammonium reversibly blocked the channel whereas 4-aminopyridine had no effect. The characteristics exhibited by this channel indicate that a Ca2+-activated K+ channel is present in the membrane of astrocytes grown in culture. These results, combined with previous evidence for a voltage dependent Ca2+ channel, suggest a dynamic role for glial cells in controlling excitability in the central nervous system. Influx of Ca2+ upon depolarization would increase the membrane permeability to K+ and could increase the "buffering" capacity of glial cells for extracellular K+.