We have developed a technique that allows mRNA molecules to be localized within cells at extremely high (nanometer) resolution. Using a cDNA for glial fibrillary acidic protein (GFAP forms lOnm diameter intermediate filaments), we have localized the mRNAGFAP in three different cell types (optic nerve astrocytes, retinal astrocytes and retinal Miiller cells). One retina was experimentally detached to stimulate GFAP expression in retinal Miiller cells. Cells containing GFAP were identified by immunoelectron microscopy. Cat retinas and optic nerves (n=7) were fixed with 2% or 4% paraformaldehyde for 1.0 hr, dehydrated with dimethylformamide, and embedded in Lowicryl K4M resin. Thin sections, placed on nickel grids, were probed with biotinylated cDNAGFAP (overnight at 37°C) followed by streptavidin-gold conjugate (1.0 hr, 23°C).In each of the three cell types that contain GFAP, the localization of the mRNAGFAP was the same. In the nuclei gold spheres were localized over amorphous, electron-dense regions within the euchromatin (presumably precursor mRNA).