Immunocytochemical studies and immunoblotting analysis demonstrated that there exists the StAR protein in bovine adrenal fasciculata cells, and ACTH activated expression of the StAR protein. Then roles of intracellular signal transduction systems in the regulation of expression of the StAR protein were studied. The addition of Bt2cAMP and forskolin, or phorbol ester plus calcium ionophore 23187 activated expression of the StAR protein as well as cortisol production, suggesting that cyclic AMP- or protein kinase C-dependent process plays a crucial role in the regulation of expression of the StAR protein. Activating effects of ACTH which activates cyclic AMP formation on the StAR protein and cortisol production were inhibited by pretreatment with calphostin C which is a protein kinase C inhibitor, suggesting that ACTH enhances expression of the StAR protein possibly via both of two signal transduction systems such as cyclic AMP- and protein kinase C-dependent processes.