The aim of the present paper is to point out the complexity of ACTH action in glomerulosa cells of the adrenal cortex. We demonstrate that the increase in cAMP production induced by ACTH is the result of a balance between activation of adenylyl cyclase and direct modulation of a PDE2 phosphodiestease activity, an effect mediated by inhibition of cGMP content. Moreover, Ca2+ is essential for cAMP production and aldosterone secretion, but its exact primary action is not clearly determined. We recently described that ACTH activated a chloride channel, via the Ras protein, which can be involved in steroidogenesis. ACTH also increases tyrosine phosphorylation of several proteins. These data, together with those of phospholipase C activation, indicate that ACTH action in the adrenal is complex, and most certainly not limited to cAMP production, in particular for the low concentrations of the hormone. Some years ago, cAMP was considered to be the unique second messenger of ACTH action; now it becomes more and more evident that ACTH triggers complex signaling pathways using several second messengers in a closely interacting way. The most predominant point is that these signals are observed for low concentrations of ACTH.