Whole peripheral venous blood from normal men and women, and from females with idiopathic hirsutism was incubated in vitro with labelled testosterone, androstenedione and dehydroepiandrosterone. Epitestosterone was formed consistently from added testosterone, in some cases from androstenedione but not from dehydroepiandrosterone. The rate of formation of epitestosterone from testosterone by blood of normal men and women is similar, whereas the rate of formation in blood from female idiopathic hirsutes was several fold greater. In a similar manner, normal human adrenal tissue also formed epitestosterone from added testosterone but not from androstenedione nor dehydroepiandrosterone. These results suggest that the origin of urinary epitestosterone can be the resultant of both peripheral production and endocrine gland secretion.