Adrenal cell-differentiated functions and, therefore, their steroidogenic capacity can be regulated in an opposite direction by insulin-like growth factor I (IGF-I) and transforming growth factor-β<sub>1</sub> (TGFβ<sub>1</sub>). The enhanced steroidogenic responsiveness of bovine and ovine adrenal cells treated with IGF-I can be explained by its positive effects on the number of corticotropin (ACTH) and angiotensin II (A-II) receptors associated with an increase in the α<sub>s</sub> and α<sub>i</sub> subunits of G proteins but also by its effects on several steps of the steroidogenic pathway. In contrast, TGFβ<sub>1</sub> is a potent inhibitor of differentiated functions of both bovine and ovine adrenal cells. TGFβ<sub>1</sub> reduces ACTH and A-II receptor number, inhibits cAMP formation, and decreases several steroidogenic enzyme activities. The physiological role of these peptides in adrenal cells is strengthened by the fact that both are synthetized and secreted by these cells and that their secretion can be regulated by the specific hormones, ACTH and A-II.