Using a new ACTH-immunoradiometric assay (IRMA), we measured plasma ACTH levels in the basal states and during CRH test in normal subjects and the patients with hypothalamo-pituitary disorders. The basal levels of plasma ACTH in 76 normal young (25-45 yr) and 140 elderly (60-85 yr) subjects were 23.1 +/- 13.6, and 17.5 +/- 11.2 pg/ml, respectively. The plasma ACTH levels were less than detection limit (5 pg/ml) in 3 patients with isolated ACTH deficiency, and less than 10 pg/ml in 6 of 7 patients with hypopituitarism. A significant correlation was observed between the basal levels of plasma ACTH and of cortisol in two age groups, with almost the same regression line, showing no age-related decline in the plasma levels of ACTH and cortisol. In 2 normal subjects and 2 patients with Cushing's disease, synchronized secretions of ACTH and cortisol were observed between 0800h and 1800h. In normal subjects and the patients with pituitary disorders, a significant correlation was observed between the Area Under the Curve's for plasma ACTH and cortisol during the CRH test. The correlation constant was higher in normal subjects, but lower in the patients with acromegaly, non-functioning pituitary tumor, and Cushing's disease in this order, suggesting low sensitivity of the pituitary-adrenal axis in these patients. These results suggest that the ACTH-IRMA kit provide reliable data for clinical investigation, and that the secretions of ACTH and cortisol correlate each other in basal states and during the CRH test in the patients with pituitary disorders as well as in normal subjects.