Recent evidence has been presented that a major fraction of plasma testosterone in the adult female is derived from peripheral conversion of plasma androstenedione. This conclusion is based upon the demonstration of a considerable conversion ratio (15%) from infused 3H-androstenedione to its conversion product testosterone and the finding of relatively large amounts (0.140 μg/100 ml) of androstenedione in female plasma (1). The back conversion of both dehydroisoandrosterone and testosterone to plasma androstenedione is minimal (2), suggesting that androstenedione must be a major secretory product of the ovary and/or adrenal cortex. It has been calculated that, since no more than one third of testosterone in female plasma can arise from direct secretion, the ratio of androstenedione to testosterone in ovarian venous plasma should be about 20:1 (1).