The authors examined the dose response relationship between plasma ACTH and cortisol concentrations after the administration of various doses of ACTH1-24 (0.025 micrograms, 0.125 micrograms, 0.25 micrograms, 1 microgram, 250 micrograms) in dexamethasone-suppressed normal volunteers. A logarithmic dose-response relationship between the dose of ACTH administered and plasma cortisol concentration was found. Although there was considerable variability in plasma ACTH concentrations, there was, however, a definite correlation between area under the curve for ACTH and area under the curve for cortisol after the various doses of ACTH.