1. An examination has been made of the free and glucuronide-conjugated corticosteroids in the plasma of normal controls and of patients before and after hypophysectomy. Quantitative measure of the free plasma corticosteroid level 72 hr after withholding cortisone from a hypophysectomized patient (cortisone withdrawal test) is a useful index of completeness of hypophysectomy. Qualitative examination failed to detect corticosteroids in these patients. 2. In all plasmas examined (controls and patients in resting state and after adrenocorticotrophic hormone (ACTH)) the main component of the free corticosteroid fraction is cortisol. In 50% of the plasma samples cortisone was detected. Even after ACTH only occasional traces of corticosterone were detected by the method used in this study. 3. In the glucuronide-conjugated fraction of plasma from subjects in the resting state three compounds, concluded to be tetrahydrocortisol (THF)†, tetrahydrocortisone, and allo-THF†, were regularly seen. After ACTH other material was detected in this fraction which is considered to be tetrahydrocorticosterone (THB), tetrahydro 11-deoxycortisol (THS) and allo-THB. Administration of oral cortisone to hypophysectomized patients did not increase the amounts of THB, THS or allo-THB in the plasma.