It is now apparent that in men and in postmenopausal women, estrogens have important physiological and pathophysiological roles. However, importantly, these actions are at a ocal level, namely paracrine, autocrine, and even 'intracrine' rather than endocrine in the classical sense. Thus for example local estrogen biosynthesis in the bones of men plays a hitherto unsuspected role in the maintenance of bone mineralization and in epiphyseal fusion; and in the testes, estrogen is essential for male germ cell development. On the other hand, in postmenopausal women, the mesenchymal cells of the breast are the major source of estrogen responsible for breast cancer development. This realization points to the importance of circulating C19 precursors in the maintenance of adequate estrogen biosynthesis in extragonadal sites and suggests the possibility of new therapies to block estrogen synthesis in a tissue-specific fashion.