Localization and regulation of mineralocorticoid receptor (MR) heteronuclear RNA (hnRNA) was assessed in rat hippocampus using an intron-directed in situ hybridization approach. The presence of hnRNA in the cell nucleus reflects recent gene transcriptional events and can be used as an index of neuronal transcriptional activation or inhibition. In the present study, sections incubated with an MR intron probe labeled cells in all regions known to contain MR mRNA. Signal generated by the intron probe was localized specifically to the nuclear compartment, consistent with recognition of hnRNA. Analysis of distribution across hippocampus indicated that MR hnRNA was particularly abundant in dentate gyrus (DG). In contrast, MR mRNA shows similar levels of expression across all hippocampal subfields. Exposure of animals to acute restraint markedly reduced MR hnRNA levels in CA1 and DG 60 min and 120 min following stress exposure, consistent with reduced gene transcription. These data support the hypothesis that stress-induced glucocorticoid secretion can rapidly affect transcription of steroid receptor genes. MR mRNA levels did not decrease over comparable time periods, suggesting either a significant lag-time between transcriptional changes and changes in cytoplasmic mRNA pools or stimulus-driven alterations in post-transcriptional RNA processing.