This electron micrograph demonstrates intense NR2A immunogold localization between an axon terminal(ax) and dendritic spine(sp) forming an asymmetric (Type 1) synapse on the dendrites of pyramidal cells in CA1 of monkey hippocampus. The majority of the gold particles, each of which probably represents an individual receptor, are associated with the postsynaptic specialization of the dendritic spine and synaptic cleft, and thus are in a position to mediate NMDA receptor activity at this particular synapse. In addition, two particles are clearly associated with the presynaptic axon terminal, suggesting that NR2A may also participate in autoreceptors that modulate glutamate release presynaptically. Thus, with this approach we can resolve the molecular constituents of the synapse and quantify their relative distribution in specific regions of the synapse. The boxed area is represented at higher magnification in lower left corner (Scale bar = 0.13µm). The author thanks William Janssen and Prabhakar Vissavajjhalla for providing this illustration.