We performed an electron microscopic study in layers II-III of S-1 in rats, using postembedding immunogold histochemistry to compare the synaptic distribution of N-methyl D-aspartate (NMDA) receptors (assessed with an antibody for the NMDAR1 subunit) with that of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors (assessed with an antibody for the GluR2/3 subunit). Labeling for each receptor was concentrated at active zones of asymmetric synapses. Analysis of the tangential position of gold particles along the postsynaptic active zone revealed that NMDA receptors were at highest concentration in the middle of the synaptic apposition, whereas AMPA receptors were concentrated in an annulus away from its center. These data support the view that the two types of receptors are anchored by distinct subsynaptic assemblies, and raise the possibility of independent synaptic microdomains.