A growing pool of evidence suggests that astrocytes are a heterogeneous population of cells. Several characteristics seem to differentiate them: electrophysiological membrane properties, expression of glutamate/GABA receptors, glutamate transporters and gap junctional coupling. The coupled astrocytes form functional networks during the first two postnatal weeks, but how and at what rate these networks are formed is still unknown. In this study we have addressed this question by birth-dating cells via injection of the cell division marker BrdU in rat pups at postnatal day 2 (P2) and P7. Subsequently, astrocytic networks in acutely prepared hippocampal slices from animals of different ages were filled with the tracer molecule neurobiotin during whole cell patch-clamp recordings. BrdU and neurobiotin were visualised histochemically and immunohistochemically. Analysis of the stainings showed that the ratio of BrdU labelled cells in networks was constant from two to ten days after BrdU injection, but was significantly higher in rats that had been injected P2 as compared with P7. When synaptic responses were investigated in astrocytes it became apparent that both electrophysiologically passive and complex astrocytes can exhibit ionotropic glutamate/GABA receptor mediated currents, but that only electrophysiologically passive astrocytes exhibit glutamate transporter currents. In conclusion, the results show that less than 2 days is needed for a newborn astrocyte to be integrated in a network and that astrocytic subtypes should not be classified on the basis of gap junctional coupling or expression of voltage gated ion channels.