The maintenance of optimal steroidogenesis in adrenocortical cells primarily depends on the chronic action of ACTH to promote the synthesis of the various steroid-metabolizing cytochrome P-450 enzymes. In the steroidogenic pathway, 17 alpha-hydroxylase cytochrome P-450 (P-450(17) alpha) is a key enzyme controlling the formation of cortisol and androgens. Recently, we demonstrated that transforming growth factor-beta (TGF beta) is a potent inhibitor of steroid production in ovine adrenocortical cells. In the present study we used a polyclonal antibody to P450(17) alpha to determine adrenal cell P-450(17) alpha enzyme content by Western analysis. In addition, we used a cDNA probe encoding for bovine P-450(17) alpha mRNA to determine levels of P-450(17) alpha mRNA in sheep ovarian adrenocortical cells in primary culture. When cells were cultured in a serum-free medium in the presence of ACTH for 48 h, P-450(17) alpha activity, enzyme content, and mRNA levels for P-450(17) alpha increased by 3- to more than 10-fold. TGF beta decreased the basal level and completely blocked the stimulatory action of ACTH on P-450(17) alpha enzyme activity. The effects of TGF beta on P-450(17) alpha enzyme content and mRNA levels were manifested in a dose-dependent manner, with maximal inhibition observed using 1 ng/ml TGF beta. Importantly, the inhibitory effects of TGF beta on P-450(17) alpha were not overcome by (Bu)2cAMP. These findings indicate that TGF beta is a potent negative regulator of P-450, and the inhibitory action appears to be at the level of P-450(17) alpha gene expression. The ability of TGF beta to suppress the positive stimulatory action of ACTH suggests that TGF beta could play a role in determining the pathway of steroidogenesis and, thereby, the specific steroids secreted by adrenocortical cells.