The relative contribution of voltage activated Na+ and K+ currents to the whole cell current pattern of hippocampal glial cells was analyzed and compared during different stages of postnatal maturation. The patch-clamp technique was applied to identified cells in thin brain slices obtained from animals between postnatal day 5 and 35 (p5-35). We focused on a subpopulation of glial cells in the CA1 stratum radiatum which most probably represents a pool of immature astrocytes, termed "complex" cells. These cells could not be labelled by O1/O4 antibodies, but some of the older cells were positively stained for glial fibrillary acidic protein (GFAP). In the early postnatal days, the current pattern of the "complex" cells was dominated by two types of K+ outward currents: a delayed rectifier and a transient component. In addition, all cells expressed significant tetrodotoxin (TTX)-sensitive Na+ currents. During maturation, the contribution of delayed rectifier and A-type currents significantly decreased. Furthermore, almost all cells after p20 lacked Na+ currents. This down-regulation of voltage gated Na+ and K+ outward currents was accompanied by a substantial increase in passive and inward rectifier K+ conductances. We found increasing evidence of electrical coupling between the "complex" cells with continued development. It is concluded that these developmental changes in the electrophysiological properties of "complex" glial cells could be jointly responsible for the well known impaired K+ homeostasis in the early postnatal hippocampus.