Intracellular recordings were made from astrocytes of the hippocampal CA1 region in rat slice preparations. A single stimulus at the Schaffer collaterals with high intensities (>/=10 V), but not low intensities (<10 V), induced a slow depolarization similar to the evoked postsynaptic potentials observed in hippocampal CA1 neurons. The slow depolarization was almost abolished in the presence of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate/kainate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (20 microM) and N-methyl-D-aspartate receptor antagonist DL-2-amino-5-phosphonopentanoic acid (100 microM). Tetrodotoxin (0.5 microM) or Co(2+) (2 mM) also abolished this slow depolarization. The slow depolarization reversed its polarity at -8 mV in the presence of tetraethylammonium (20 mM). These results suggest that the synaptically released glutamate (Glu) induces depolarization, which is mainly mediated by ionotropic Glu receptors, in astrocytes in the hippocampal CA1 region.