Microglial cells were isolated from rat cerebral cortex, and kainate (KA)-induced inward current was measured at a holding potential of −40 or −60 mV. 6-Cyano-7-nitroquinoxaline-2, 3-dione-sensitive KA-induced currents increased with increasing KA concentration. The half-activation concentration and Hill coefficient were 3.3 × 10⁻⁴m and 1.4, respectively. Although glutamate (Glu) and AMPA-induced currents were much smaller than that induced by KA, all KA-, Glu-, and AMPA-induced currents were greatly and consistently enhanced in the presence of cyclothiazide (CTZ). On the other hand, KA-induced currents were much less sensitive to potentiation by concanavain A, suggesting that the KA-induced response in rat microglia is predominantly mediated by AMPA-preferring receptors (subunits GluR1–GluR4). The current–voltage relationships of KA- and AMPA–CTZ-induced currents were almost linear or slightly outward rectifying. The reversal potential of KA-induced current shifted to negative potentials (from +4 to −40 mV) on switching from high Na⁺ to high Ca²⁺ external solution, indicating the low Ca²⁺ permeability through the AMPA–KA receptor channel complexes. AMPA–KA receptor expression was studied with immunohistochemistry and reverse transcription-PCR, from which GluR2, GluR3, GluR4, and GluR5 were identified. Lower levels of mRNAs for GluR7 and KA-1–KA-2 were also indicated. Finally, activation of these receptors with KA or Glu significantly enhanced the production of tumor necrosis factor-α. These results suggest that primary cultured rat microglia possesses functional Glu receptor, which may mediate neuron to microglia communication in the physiological and pathological states.