Antagonists of pituitary trophic hormones may act either by inhibiting the synthesis and release of the hormone, by interfering with the interaction of the trophic hormone with its target tissue, or by inhibiting the intracellular response of the target cell. Taking the adrenocorticotrophin (ACTH) – adrenal cortical cell system some inhibitors of the “trophic” and “acute steroidogenic” responses of the cortical cells to ACTH are described. Finally the problems associated with using inhibitors of specific intracellular functions in the whole animal are considered.