Ligand-dependent nuclear hormone receptors (NRs), such as retinoic acid and thyroid hormone receptors, play critical roles in diverse aspects of development. They enhance or repress transcription by recruiting an array of coactivator and corepressor proteins, which function as signaling intermediates between the NRs and the basal transcriptional machinery. To study the possible involvement of these cofactors on tissue-specific regulation of gene expression by NRs, we examined the expression of the coactivator SRC-1 mRNA during mouse embryogenesis by in situ hybridization (ISH). 35S-labeled riboprobe specific for SRC-1 mRNA was used for analysis. The distribution of this transcript was studied from 8.5 to 18.5 embryonic days (E8.5-E18.5) and in postnatal day 15 (P15). The SRC-1 transcript was largely ubiquitously expressed, even on E8.5. At E14.5 and E18.5, highest levels of SRC-1 transcript was found in the olfactory epithelium. Significant SRC-1 hybridization signal was also detected in the neocortex, anterior pituitary and heart. We conclude that (1) SRC-1 mRNA is widely expressed in the developing embryo, and (2) SRC-1 mRNA is expressed at the highest level in the olfactory epithelium, suggesting that this coactivator may be involved in the development and/or function of the olfactory system.