GABA_A receptor-mediated inhibitory postsynaptic currents (IPSCs) modify excitatory synaptic transmission by altering the activity of principal neurons. The plasticity of the inhibitory synapses has not been well characterized, comparing with that of the excitatory one. Postsynaptic depolarization-induced changes of GABA_Aergic synaptic transmission have been reported, but both facilitation and inhibition were observed, and mechanisms of the plasticity have not been elucidated. In the present experiments, effect of the postsynaptic depolarization on spontaneous GABA_Aergic synaptic transmission was examined in acute slices of neonatal rat hippocampus. Using whole cell patch-clamp recording method, spontaneous GABA_Aergic IPSCs (sIPSCs) were recorded from CA3 neurons. Postsynaptic depolarization alone did not cause marked alteration of the frequency or amplitude of sIPSCs. Facilitation of the transmitter release by presynaptic depolarization increased frequency of the sIPSCs. Simultaneous activation of presynaptic and postsynaptic neurons, however, induced transient inhibition of the frequency of the sIPSCs. Though specific agonist of metabotropic glutamate receptors exogenously applied during postsynaptic depolarization did not mimic the inhibition of the sIPSCs, antagonists of the receptors partially suppressed the inhibition caused by the simultaneous pre- and post-synaptic stimulation. These results suggest that postsynaptic depolarization and simultaneous facilitation of presynaptic release of glutamate transiently inhibited the sIPSCs.