The steroidogenic acute regulatory (StAR) protein is a rate-limiting factor in steroid hormone production. The StAR protein plays a role in the movement of cholesterol from the outer membrane to the inner membrane, where cholesterol side chain cleavage enzyme exists. Dioxins, which may act as 'endocrine disruptors', mimic and antagonize endogenous hormone actions in vivo. Although the mechanism of endocrine disruption is not clear, the actions of dioxins are known to be mediated by binding to the arylhydrocarbon receptor (AhR), and it is known that dioxins act as transcription factors to endocrine-associated gene expression. In the present study, we examined the effect of the AhR on the human StAR gene promoter, and we clarified the action mechanisms of environmental endocrine disruptors. We transfected constructs containing the human StAR gene promoter sequences pGL(2) 1.3-kb StAR (nt -1293 to +39) into mouse Y-1 adrenal tumor cells and measured the promoter activity of the StAR gene. With the addition of beta-napthoflavone (betaNF), which is a ligand of AhR, to the culture medium, the activity of the StAR gene promoter increased significantly (P<0.05), and with the addition of 1 microM of betaNF, it became maximum (3.1+/-0.6-fold higher than the control value). When the AhR and ARNT were co-transfected together in Y-1 cells or human adrenocortical carcinoma H295R cells, the promoter activity of the StAR gene significantly (P<0.05) increased, to a level 1.4+/-0.01-fold higher in Y-1 cells and to a level 1.6+/-0.04-fold higher in H295R cells than the control level, when 1 microM of betaNF was added. We examined the effect of induction of cAMP with transfection with AhR or ARNT. With the addition of 1 mM 8-Br-cAMP, there were no differences between the StAR gene promoter activities in the group in which AhR and ARNT was introduced and in the group in which they were not introduced. The results suggest that AhR plays a role in the promoter activity of the human StAR gene and that the effect of AhR on StAR gene expression may cause a disturbance to the human endocrine system.