The tubulin-binding protein gephyrin, which anchors the inhibitory glycine receptor (GlyR) at postsynaptic sites, decorates GABAergic postsynaptic membranes in various brain regions, and postsynaptic gephyrin clusters are absent from cortical cultures of mice deficient for the GABA(A) receptor gamma2 subunit. Here, we investigated the postsynaptic clustering of GABA(A) receptors in gephyrin knock-out (geph -/-) mice. Both in brain sections and cultured hippocampal neurons derived from geph -/- mice, synaptic GABA(A) receptor clusters containing either the gamma2 or the alpha2 subunit were absent, whereas glutamate receptor subunits were normally localized at postsynaptic sites. Western blot analysis and electrophysiological recording revealed that normal levels of functional GABA(A) receptors are expressed in geph -/- neurons, however the pool size of intracellular GABA(A) receptors appeared increased in the mutant cells. Thus, gephyrin is required for the synaptic localization of GlyRs and GABA(A) receptors containing the gamma2 and/or alpha2 subunits but not for the targeting of these receptors to the neuronal plasma membrane. In addition, gephyrin may be important for efficient membrane insertion and/or metabolic stabilization of inhibitory receptors at developing postsynaptic sites.