Multipotential progenitor cells grown from central nervous system (CNS) tissues in defined media supplemented with epidermal growth factor (EGF), when attached to a suitable substratum, differentiate to express neural and glial histochemical markers and morphologies. To assess the functional characteristics of such cells, expression of voltage-gated Na⁺and K⁺currents (I_Na,I_K) was studied by whole-cell patch clamp methods in progenitors raised from postnatal rat forebrain. Undifferentiated cells were acutely dissociated from proliferative “spheres,” and differentiated cells were studied 1–25 days after plating spheres onto polylysine/laminin-treated coverslips.I_NaandI_Kwere detected together in 58%,I_Naalone in 11%, andI_Kalone in 19% of differentiated cells recorded with K⁺-containing pipettes. With internal Cs⁺(to isolateI_Na),I_Naup to 45 pA/pF was observed in some cells within 1 day after plating.I_Naranged up to 150 pA/pF subsequently. Overall, 84% of cells expressedI_Na, with an average of 38 pA/pF.I_Nahad fast kinetics, as in neurons, but steady-state inactivation curves were strongly negative, resembling those of glialI_Na. Inward tail currents sensitive to [K⁺]_outwere observed upon repolarization after the 10-ms test pulse with internal Cs⁺, indicating the expression of K⁺channels in 82% of cells. In contrast to the substantial currents observed in differentiating cells, little or noI_NaorI_K-tail currents were detected in recordings from cells acutely dissociated from spheres. Thus, in the presence of EGF, ionic currents develop early during differentiation induced by attachment to an appropriate substratum. Cells switched from EGF to basic fibroblast growth factor (bFGF) when plated onto coverslips showed greatly reduced proliferation and developed less neuron-like morphologies than cells plated in the presence of EGF.I_Nawas observed in only 53% of bFGF-treated cells, with an average of 9 pA/pF. Thus, in contrast to reports that bFGF promotes neuronal differentiation in some CNS progenitor populations, our EGF-generated postnatal rat CNS progenitors do not develop neuronal characteristics when switched to medium containing bFGF. Thus, differentiated CNS progenitors can express a mix of neuronal and glial molecular, morphological, and electrophysiological properties that can be modified by culture conditions.