Glial cells from different brain regions and species are depolarized by the neurotransmitter glutamate. The depolarization or, if voltage-clamped at the resting membrane potential, the inward current induced by glutamate could be due either to activation of receptorcoupled ion channels or electrogenic uptake of the transmitter. In the present study we applied the patch-clamp technique in the whole-cell recording mode to analyze glutamate-induced currents in cultured astrocytes from rat cerebral hemispheres. At the resting membrane potential, glutamate induced an inward current ranging from 40 to 300 PA. This current decreased in size with depolarization and reversed at about 0 mV. The resulting current-to-voltage curve was linear and depended strongly on the transmembrane Na+ but not on the Ca++ or C1-gradient. In the presence of glutamate, current noise increased at potentials positive or negative from the reversal potential indicating that ionic channels are activated by glutamate. Both kainate and quisqualate mimicked the effect of glutamate. We conclude that glutamate opens a Na+/K+ channel in cultured astrocytes because of activation of a receptor which shares many properties with the neuronal kainate/quisqualate receptor.