Glial cells, which outnumber neurones in the central nervous system, have traditionally been considered to be electrically inexcitable and to play only a passive role in the electrical activity of the brain1. Recent reports have demonstrated, however, that certain glial cells, when maintained in primary culture, possess voltage-dependent ion channels2-4. It remains to be demonstrated whether these channels are also present in glial cells in vivo. I show here that Müller cells, the principal glial cells of the vertebrate retina, can generate 'Ca2+spikes' in freshly excised slices of retinal tissue. In addition, voltage-clamp studies of enzymatically dissociated Müller cells demonstrate the presence of four types of voltage-dependent ion channels: A Ca2+ channel, a Ca 2+-activated K+ channel, a fast-inactivating (type A) K+ channel and an inward-rectifying K+ channel. Currents generated by these voltage-dependent channels may enhance the ability of Müller cells to regulate extracellular K+ levels in the retina and may be involved in the generation of the electroretinogram.