Expression of N-methyl d-aspartate (NMDA) receptor-dependent homosynaptic long term depression at synapses in the hippocampus and neocortex requires the persistent dephosphorylation of postsynaptic protein kinase A substrates. An attractive mechanism for expression of long term depression is the loss of surface AMPA (alpha-amino-3-hydroxy-5-methylisoxazale-4-propionate) receptors at synapses. Here we show that a threshold level of NMDA receptor activation must be exceeded to trigger a stable loss of AMPA receptors from the surface of cultured hippocampal neurons. NMDA also causes displacement of protein kinase A from the synapse, and inhibiting protein kinase A (PKA) activity mimics the NMDA-induced loss of surface AMPA receptors. PKA is targeted to the synapse by an interaction with the A kinase-anchoring protein, AKAP79/150. Disruption of the PKA-AKAP interaction is sufficient to cause a long-lasting reduction in synaptic AMPA receptors in cultured neurons. In addition, we demonstrate in hippocampal slices that displacement of PKA from AKADs occludes synaptically induced long term depression. These data indicate that synaptic anchoring of PKA through association with AKAPs plays an important role in the regulation of AMPA receptor surface expression and synaptic plasticity.