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Astrocytes are a major cell type in the mammalian central nervous system (CNS), yet their functions remain uncertain. There are two principal classes of these glial cells--protoplasmic astrocytes, found mainly in grey matter, and fibrous astrocytes, which occur mainly in white matter. Recently, these two types of astrocytes have been distinguished in cultures of developing CNS and have been shown to be biochemically distinct. Because both types contain large numbers of glial filaments in vitro and hence appear 'fibrous', we will refer to them as type 1 (protoplasmic) and type 2 (fibrous) astrocytes. Most type 2 astrocytes in culture share several properties with neurones; for example, they have a process-bearing morphology and bind tetanus toxin and the monoclonal antibody A2B5, both of which recognize specific gangliosides and were initially considered to be neurone-specific markers in the CNS. We have therefore investigated whether type 2 astrocytes also share electrophysiological properties with neurones. Using intracellular microelectrode and 'whole-cell' (patch-clamp) recording techniques, we have now found that both type 1 and type 2 astrocytes in culture have time- and voltage-dependent potassium ion conductances which, until recently, were considered to be confined largely to electrically excitable cells.