The glutamatergic system mediates the vast majority of excitatory synaptic activity and plays a crucial role in information processing in the neural circuits. In a process of synaptic transmission, glutamate released from presynaptic terminals activates α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/kinate receptors (AMPARs), which produce the primary component of excitatory postsynaptic currents (EPSCs). Therefore, spontaneous activity is assumed to decrease drastically when AMPARs are blocked. We monitored spiking activity of hippocampal CA3 neurons using functional multineuron calcium imaging (fMCI). In contrast to our expectation, spontaneous activity in hippocampal local circuits was maintained even when 6-cyano-2,3-dihydroxy-7-nitro-quinoxaline (CNQX), an competitive AMPAR antagonist, was applied. Moreover, CNQX did not perturb the regularity of the inter-spike-interval of individual neurons or the event frequency of synchronized activities, which indicates that the pattern of neuronal activity was preserved. To elucidate the synaptic mechanisms, we recorded spontaneous EPSCs and inhibitory postsynaptic currents (IPSCs). The mean amplitudes of EPSCs as well as IPSCs were reduced by CNQX, suggesting that the net balance between excitatory and inhibitory inputs was preserved after blockade of AMPARs. These data show that hippocampal network activity persists despite the reduction of synaptic activity.