The rise in plasma corticosteroid concentration after the administration of corticotrophin (ACTH) is an index of the functional reserve of the adrenal cortex (Eik-Nes, Sandberg, Nelson, Tyler, & Samuels, 1954). A synthetic polypeptide β1–24 (Synacthen, Ciba) containing the amino acids 1–24 of the 39 found in pituitary ACTH (Kappeler & Schwyzer, 1961) is as active in stimulating the adrenal as corticotrophin (Landon, James, Cryer, Wynn & Frankland, 1964; Ohlsen & Hökfelt, 1965; Wood, Frankland, James & Landon, 1965). Wood et al. (1965) have shown that the rise in plasma corticosteroids (Mattingly, 1962) 30 min. after the i.m. administration of 250 μg. Synacthen can be used as a measure of adrenocortical reserve. We report our experience with this test carried out between 8 a.m. and 10 a.m. in two groups of hospital patients. Group I comprised 21 women and nine men, aged from 20 to 73 yr. (mean age 46