6-Cyano-7-nitroquinoxaline-2,3-dione (CNQX) is widely used as an antagonist on non-NMDA glutamate receptors. However, several studies have shown that CNQX increases the spontaneous inhibitory postsynaptic current frequency at hippocampal pyramidal neurons and cerebellar granule cells. Dentate granule cells are known to be another distinctive type of principal neurons in hippocampus, and receive dense synaptic input from hilar interneurons. Thus, we examined the effects of CNQX on the dentate granule cells and hilar interneurons with whole-cell recording. CNQX increased the frequency of GABAergic spontaneous postsynaptic currents (sPSCs) on the granule cells, and increased the resting potential and the action potential frequency of the hilar interneurons. These increases were not observed with other glutamate receptor antagonists. The increases in sPSC frequency may be caused by the depolarization and the action potentials of the interneurons.