SUMMARYFollowing intravenous injection of a pharmacological dose of dexamethasone, there was a prompt fall in both C‐terminal and N‐terminal plasma immunoreactive ACTH levels in nineteen subjects with Addison's disease. Intravenous infusion of cortisol, sufficient to give physiological plasma levels in three of these subjects, induced a fall in plasma C‐terminal immunoreactive ACTH concentration only when plasma corticosteroid levels exceeded 20–25 μg/IOO ml. Thus although negative feedback control of ACTH secretion was still operative in these subjects, it seems likely that this mechanism of ACTH regulation had diminished sensitivity compared with that found by other workers in normal ind