Pharmacological, physiological, and autoradiographic studies have suggested the presence of dopamine receptors in the adrenal gland. Dopaminergic ligands have been shown to modulate adrenocortical aldosterone biosynthesis and secretion as well as adrenomedullary catecholamine production and release. Using a combination of light microscopic immunochemistry and in situ amplification and hybridization, the present study sought to determine the site-specific expression of the recently cloned D1A receptor subtype in rat adrenal gland. Light microscopic immunohistochemistry was conducted using polyclonal antisera raised to the putative rat D1A receptor. Immunoreactive product was detected using an avidin-biotin immunoperoxidase method. D1A receptor messenger RNA (mRNA) was detected using a transcription-based isothermal in situ amplification and hybridization approach using receptor-specific mRNA oligonucleotide probes. The amplified product was localized using an alkaline phosphatase 4-nitro blue tetrazolium chloride/5-bromo-4-chloro-3-indolyl-phosphate technique. This combined experimental approach, using both receptor subtype-selective antibodies and oligonucleotide probes, allows for the site-specific localization of the D1A receptor subtype, which would otherwise not be possible with the pharmacological methods currently available. The D1A receptor protein and mRNA were expressed solely in the zona glomerulosa of the rat adrenal gland, with no signal evident in any of the other cortical layers or in the medulla. Such a distribution raises the possibility that the D1A receptor subtype could modulate, at least in part, some of the known effects of dopamine on aldosterone secretion.