Desensitization of alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA) receptors is thought to shape the synaptic response and act as a neuroprotective mechanism at central synapses, but the molecular mechanism underlying desensitization is poorly understood. We found that replacing the glutamate binding domain S1 of GluR3 (an AMPA receptor) with S1 of GluR6 (a kainate receptor) resulted in a fully active but completely nondesensitizing receptor. Smaller substitutions within S1 identified, besides two additional modulatory regions, a single exchange, L507Y, as is required and sufficient for the block of desensitization. This phenotype was specific for AMPA receptors and required an aromatic residue at this position. L507 lies between two residues (T504 and R509) that form part of the glutamate binding site. The physical proximity of these residues, which are involved in binding and gating, suggests they may form part of the link between these two events.