We have employed polyclonal antibodies to a peptide sequence of bovine steroidogenic acute regulatory (StAR) protein and human placental 3 beta-hydroxysteroid dehydrogenase (3 beta-HSD) to determine the localisation and distribution of these proteins in rat and bovine adrenal glands. Immunohistochemical staining demonstrated the presence of StAR protein in the zona glomerulosa (ZG), zona fasciculata (ZF), zona reticularis (ZR) and in the medulla of both species. For 3 beta-HSD, immunostaining was observed in the ZG, ZF and ZR of the rat adrenal and was absent in the medulla. Immunoblotting experiments showed intense bands for StAR protein (30 kDa, 37 kDa) in the mitochondria of bovine ZG, ZF and medulla and a less intense band (30 kDa) in the microsomes. In rat ZG and ZF/R mitochondria only the 30 kDa protein was present. For 3 beta-HSD, an intense band (42 kDa) was found in microsomes and mitochondria of rat and bovine ZG and ZFR. A very faint signal for 3 beta-HSD was seen in adrenal medulla. In conclusion, StAR (or a closely related) protein is present throughout the adrenal gland in rat and bovine species in contrast to 3 beta-HSD which is confined to the steroidogenic zones. The possible function of StAR protein in the adrenal medulla merits investigation.