Here, we show that N-ethylmaleimide–sensitive fusion protein (NSF) interacts directly and selectively with the intracellular C-terminal domain of the GluR2 subunit of AMPA receptors. The interaction requires all three domains of NSF but occurs between residues Lys-844 and Gln-853 of rat GluR2, with Asn-851 playing a critical role. Loading of decapeptides corresponding to the NSF-binding domain of GluR2 into rat hippocampal CA1 pyramidal neurons results in a marked, progressive decrement of AMPA receptor–mediated synaptic transmission. This reduction in synaptic transmission was also observed when an anti-NSF monoclonal antibody (mAb) was loaded into CA1 neurons. These results demonstrate a previously unsuspected direct interaction in the postsynaptic neuron between two major proteins involved in synaptic transmission and suggest a rapid NSF-dependent modulation of AMPA receptor function.