Homer is a postsynaptic density (PSD) scaffold protein that is involved in synaptic plasticity, calcium signaling and neurological disorders. Here, we use pre-embedding immunogold electron microscopy to illustrate the differential localization of three Homer gene products (Homer 1, 2, and 3) in different regions of the mouse brain. In cross-sectioned PSDs, Homer occupies a layer ?30-100nm from the postsynaptic membrane lying just beyond the dense material that defines the PSD core (?30-nm-thick). Homer is evenly distributed within the PSD area along the lateral axis, but not at the peri-PSD locations within 60nm from the edge of the PSD, where type I-metabotropic glutamate receptors (mGluR1 and 5) are concentrated. This distribution of Homer matches that of Shank, another major PSD scaffold protein, but differs from those of other two major binding partners of Homer, type I mGluR and IP3 receptors. Many PSD proteins rapidly redistribute upon acute (2min) stimulation. To determine whether Homer distribution is affected by acute stimulation, we examined its distribution in dissociated hippocampal cultures under different conditions. Both the pattern and density of label for Homer 1, the isoform that is ubiquitous in hippocampus, remained unchanged under high K(+) depolarization (90mM for 2-5min), N-methyl-d-asparic acid (NMDA) treatment (50?M for 2min), and calcium-free conditions (EGTA at 1mM for 2min). In contrast, Shank and calcium/calmodulin-dependent kinase II (CaMKII) accumulate at the PSD upon NMDA treatment, and CaMKII is excluded from the PSD complex under low calcium conditions.