A competitive binding assay using the cortical receptor from normal adrenal glands has been developed for ACTH. In this assay porcine, human or rabbit ACTH has been shown to compete with labeled porcine ACTH for the cortical receptor from ovine, rabbit or rat adrenal glands. One pg per assay tube reproducibly caused a significant decrease in percent bound. The mean plasma ACTH concentration was 75 pg/ml in normals at 0800 and 54 pg/ml in normals at 1530. Plasma ACTH was markedly elevated after oral metyrapone or during insulin hypoglycemia and was suppressed following dexamethasone. Addisonian patients consistently had elevated plasma ACTH concentrations prior to their maintenance hydrocortisone; ACTH concentrations were not consistently elevated in one patient with untreated Cushing's disease.