The electrophysiological properties of glial cells were examined in primary culture in the presence of tetraethylammonium and Ba 2+ , a treatment that reduces K + permeability of the membrane and enhances currents through voltage-dependent Ca 2+ channels. Under these conditions, glial cells showed both spontaneous action potentials and action potentials evoked by the injections of current. These responses appear to represent entry of Ba 2+ through Ca 2+ channels because they were resistant to tetrodotoxin but were blocked by Mn 2+ or Cd2+.