The current data indicate that during a woman's reproductive years, 17 beta-hydroxysteroid dehydrogenase type 1 is the major 17 beta-hydroxysteroid dehydrogenase (17HSD) involved in glandular oestradiol biosynthesis. The type 1 enzyme catalyses reduction from low-activity oestrone to high-activity oestradiol in ovarian granulosa cells and placental syncytiotrophoblasts, in which it is abundantly expressed. In addition to steroidogenic cells, 17HSD type 1 is present in certain peripheral tissues in which it reduces circulating oestrone, thus regulating the intracellular ligand supply for oestrogen receptors. Several factors and second messenger pathways are involved in the cell-specific expression of 17HSD type 1. In ovarian granulosa cells, 17HSD type 1 expression is strictly regulated by pituitary gonadotrophins, steroid hormones and growth factors, while in peripheral tissues progestins and retinoic acids, at least, affect 17HSD type 1 concentrations.