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γ-Aminobutyric acid (GABA) spillover from synaptic cleft activates extrasynaptic GABAA receptor and results in a tonic inhibition, which induces a background inhibitory effect to stabilize the membrane potential of the neuronal cells. However, the role of tonic inhibition and how it can be regulated during brain development and epileptic state remain elusive. By whole-cell patch-clamp recording on the granule cell in the dentate gyrus, we recorded tonic conductance to investigate the level of tonic inhibition in these two critical periods. According to our observation, an age-dependent increase in tonic conductance was observed. Furthermore, a change in tonic inhibition was also found in a chronic epileptic animal model, indicating that the alteration in tonic inhibition after epilepsy induction persists for a long duration to modulate neuronal activities. The present results show that tonic inhibition is altered during brain development and a chronic epileptic condition, indicating a role of the tonic inhibitory effect in both the critical periods.