The CYP11B1 gene, which encodes steroid 11 beta-monooxygenase, which is responsible for the synthesis of cortisol and corticosterone, the major glucocorticoids in mammals, is expressed specifically in the zona fasciculata of the adrenal cortex. We have analyzed the promoter region of the rat CYP11B1 gene by using a transient-expression system with adrenocortical Y1 cells and have identified a positive regulatory region. The region contained two adjacent sites for the binding of Y1-cell nuclear proteins: the binding site for an AP-1 transcription factor composed of JunD and a Fos-related protein, and the site for Ad4-binding protein (Ad4BP). The binding of the AP-1 factor to the regulatory region had a suppressive effect on that of Ad4BP in the nuclear extracts. Mutational analyses revealed that the transcriptional activation of the CYP11B1 gene promoter in Y1 cells was attributable to the AP-1 site but not to the Ad4 site. Subsequently, nuclear extracts of the zona fasciculata cells from the rat adrenal cortex were found to contain both AP-1 factor and Ad4BP, whose binding properties to the regulatory region were almost identical to those of the two factors in the Y1-cell nuclear extracts. Moreover, immunohistochemical analyses of rat adrenal cortices showed that the AP-1 factor was present in the nuclei of CYP11B1-expressing cells in the zona fasciculata but not in the nuclei of cells in the other zones. From these results, we propose that the AP-1 transcription factor found in this study plays an important role in the zone-specific expression of the CYP11B1 gene in rat adrenal cortex.