The ability of nerve cells to receive up to several thousands of synaptic inputs from other neurons provides the anatomical basis for information processing in the vertebrate brain.The formation of functional synapses involves selective clustering of neurotransmitter receptors at presumptive postsynaptic regions of the neuronal plasma membrane [1-4]. Receptor-associated proteins are believed to be crucial for this process. In spinal neurons, synaptic targeting of the inhibitory glycine receptor (GlyR) [5,6] depends on the expression of the anchoring protein gephyrin [7-9]. Here we show that the competitive GlyR antagonist strychnine and L-type Ca2+-channel blockers inhibit the accumulation of GlyR and gephyrin at postsynaptic membrane areas in cultured rat spinal neurons. Our data are consistent with a model in which GlyR activation that results in Ca sup 2+ influx is required for the clustering of gephyrin and GlyR at developing postsynaptic sites. Similar activity-driven mechanisms may be of general importance in synaptogenesis.