A B S T R A C T Techniques are described in detail for a radioimmunoassay of plasma adrenocorticotropin (ACTH) that is capable of detecting hormone in unextracted normal human plasma at 1: 5 dilution under the conditions described. The sensitivity of the assay is at the level of 1 ,qLug/ml (equivalent to 0.014 mU/100 ml).In normal subjects ACTH concentrations averaged 22 ,uug/ml (equivalent to 0.308 mU/100 ml) plasma at 8-10 a.m. In a smaller group the concentrations averaged 9.6 lqqg/ml (equivalent to 0.134 mU/100 ml) at 10-11 p.m. Although a circadian rhythm in normal subjects was not always well marked throughout the daytime hours, plasma ACTH usually fell to its lowest value in the late evening. In hospital patients who were not acutely ill, concentrations were infrequently above 100 /A/Ag/ml in the morning and usually fell to significantly lower levels in the late evening. Severely ill hospital patients occasionally exhibited a.m. concentrations above 200 /Apg/ml.In a group of subjects showing frequent spiking -of plasma 17-OHCS concentrations throughout the day parallel spiking of plasma ACTH as well was generally observed.Metyrapone produced marked increases in plasma ACTH within 24 hr in all cases and generally within 3-6 hr except when started late in the day. Dexamethasone brought about a persistent reduction in plasma ACTH in a patient under continued treatment with metyrapone.Hypoglycemia, electroshock, surgery under general anesthesia, histalog and vasopressin administration were usually followed by significant in-*creases in plasma ACTH concentration. Prior