We investigated the influence of synaptically released glutamate on postsynaptic structure by comparing the effects of deafferentation, receptor antagonists and blockers of glutamate release in hippocampal slice cultures. CA1 pyramidal cell spine density and length decreased after transection of Schaffer collaterals and after application of AMPA receptor antagonists or botulinum toxin to unlesioned cultures. Loss of spines induced by lesion or by botulinum toxin was prevented by simultaneous AMPA application. Tetrodotoxin did not affect spine density. Synaptically released glutamate thus exerts a trophic effect on spines by acting at AMPA receptors. We conclude that AMPA receptor activation by spontaneous vesicular glutamate release is sufficient to maintain dendritic spines.