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THE long-term modification of inhibitory postsynaptic potentials (IPSPs) was studied in visual cortex slices taken from developing rats. IPSPs evoked by layer IV stimulation were intracellularly recorded from layer V cells while excitatory synaptic transmission was blocked by NMDA and non-NMDA receptor antagonists. High-frequency conditioning stimulation of layer IV induced long-term potentiation of IPSPs. By contrast, long-term depression (LTD) of IPSPs was induced by the same conditioning stimulation applied while NMDA receptormediated synaptic transmission was unmasked by removing the NMDA antagonist from and adding a GABAA receptor antagonist to the medium. The LTD of IPSPs was also induced by NMDA application to the cells. The plasticity of IPSPs might explain the postnatal development of selective responsiveness of visual cortical cells.