Administration of convulsant drugs causes the rapid induction of c-fos in identified neurons within the mouse central nervous system (Morgan et al., 1987). In particular, Fos-like immunoreactivity is evident in nuclei of granule cells of the hippocampal dentate gyrus within 30 minutes of the onset of seizure. By immunoelectron microscopy, Fos antibody binding was exclusively localized to dispersed chromatin (euchromatin) of several types of projection neurons and local circuit neurons in various brain regions and especially in the dentate gyrus, 210 minutes after a single injection of Metrazol. Fos-like immunoreactivity was not detectable in the nucleolus, nor in the characteristic peripheral and nucleolus-associated heterochromatin of hippocampal granule cells. No immunostaining was observed in nuclei of glial, ependymal or endothelial cells, and no cytoplasmic reactivity was seen in any cell type. These findings support a role for Fos in stimulus-response coupling at the level of transcriptional regulation in neurons.