PSD-95 (SAP90), SAP102 and Chapsyn-110 (PSD-93) are members of the membrane-associated guanylate kinase family, and interact with N-methyl-D-aspartate (NMDA) receptor NR2A (GluRepsilon1) and NR2B (GluRepsilon2) subunits and with Shaker-type K+ channel subunits to cluster into a channel complex. In the present study, we examined their expression in developing and adult mouse brains by in situ hybridization with antisense oligonucleotide probes. PSD-95 and SAP102 mRNAs were prominently expressed at embryonic day 13 (E13) in the mantle zone of various brain regions, where NMDA receptor NR2B subunit mRNA is expressed at high levels. In the early postnatal period when active synaptogenesis takes place, both mRNAs became elevated and concentrated in the telencephalon and cerebellar granular layer, where NR2A and/or NR2B subunit mRNAs are abundantly expressed. Chapsyn-110 mRNA was, though at low levels, found over the mantle zone of embryonic brains, and the level was progressively increased in the telencephalon starting at perinatal stages. The spatial and temporal correlations in the brain in vivo suggest that the PSD-95/SAP90 protein family can interact with NMDA receptor subunits to cluster them into channel complex at both synaptic and non-synaptic sites before, during and after synaptogenic stages.