Developmental changes in ion channel expression and cell morphology were studied in glial cells with the use of whole cell patch-clamp recordings in rat [postnatal day (P)5-P50] hippocampal slices. Recordings were obtained from 234 cells, presumed to be glia, in stratum radiatum and stratum lacunosum-moleculare of the CA1 region. Of 66 recorded cells filled with Lucifer yellow, 48 stained positive for glial fibrillary acidic protein, which identified them as astrocytes. All glial cells studied were of a stellate morphology, and developmental changes primarily comprised an increase in the length and number of cell processes associated with an overall increase in cell size and membrane capacitance. Two distinct outward potassium currents could be identified: a transient 4-aminopyridine-sensitive current (Ia) and a persistent outward current sensitive to tetraethylammonium (Id). Ia activated at -40 mV, and steady-state activation and inactivation midpoints were -16 and -74 mV, respectively. Decay time constants ranged from 7 ms at -30 mV to 19 ms at +80 mV. Id activated at -30 mV. A third K+ current sensitive to cesium activated with hyperpolarizing command voltages and showed strong inward rectification. Transient, voltage-activated sodium currents (I(Na)) were tetrodotoxin sensitive (100 nM) and activated at about -40 mV, peaked at about -10 mV, and reversed at +63 mV. I(Na) was half-inactivated at -49 mV and half-activated at -19 mV. During the first 2 wk of postnatal development, the percentage of cells showing inwardly rectifying K+ current (Ir), Ia, and I(Na) increased significantly from 40% (at P5) to 90% (at P20-P50). By contrast, almost all cells independent of age expressed Id. Specific conductances for Ir (g(ir)) and Ia increased significantly between P5 and P20, concomitant with a decrease in input resistance. By contrast, specific conductance of the outwardly rectifying K+ current (g(d)) decreased threefold between P5 and P20. Specific Na+ conductance was always <1/4 of the total potassium conductance. These results indicate that CA1 hippocampal astrocytes are characterized by expression of voltage-activated Na+ channels and three types of K+ channels showing changes in their relative expression during early postnatal development: 1) the number of cells expressing Ia, Ir, and I(Na) increases significantly and 2) their specific conductance changes such that g(d), predominant at P5-P20, is gradually replaced by g(ir), the predominant conductance in adult astrocytes. Adult morphological and electrophysiological phenotypes are established at about P20. These data suggest that previous studies in which cultured or acutely isolated cells from immature or embryonic rats were used were not adequately reflecting the properties of hippocampal astrocytes in situ.