Following Selye's theory on the General Adaptation Syndrome increasing attention has been paid to the role of the adrenal glands in the defence mechanism against «stressors» The adrenocortical activity has been investigated by different methods, such as the eosinopenic reaction, the depletion of ascorbic acid or cholesterol in the adrenal cortex and estimations of the urinary excretion of 17-hydroxycorticosteroids and 17-ketosteroids (Bayliss et al., 1955, Sayers, 1948, Thorn, 1949, Uotila & Pekkarinen, 1951, Halme et al., 1957, Pekkarinen et al., 1957). Since suitable methods have been developed for the estimation of free 17-hydroxycorticosteroids (17-OHCS) in plasma (Nelson & Samuels, 1952, Peterson & Wyngaarden, 1955, Reddy et al., 1956, Silber & Bush, 1956) changes in the adrenocortical activity can easily be followed under normal and abnormal conditions. Under normal conditions there is a diurnal rhythm in the plasma 17-OHCS (Mach, 1957, Migeon et al., 1956). The maximal content is observed