Within potential target cells, the actions of physiological glucocorticoids (cortisol and corticosterone) are modulated by isoforms of the enzyme 11 beta-hydroxysteroid dehydrogenase (11 beta HSD). To date, two isoforms of 11 beta HSD have been cloned: 11 beta HSD1 acts predominantly as an NADP(H)-dependent reductase to generate active cortisol or corticosterone, and 11 beta HSD2 is a high affinity NAD(+)-dependent enzyme that catalyses the enzymatic inactivation of glucocorticoids. Whereas the regeneration of active glucocorticoids by 11 beta HSD1 has been implicated in the cellular mechanisms of pituitary function, ovulation and parturition, the enzymatic inactivation of cortisol and corticosterone by 11 beta HSD enzymes appears to be central to the protection of gonadal steroidogenesis, prevention of intra-uterine growth retardation, and lactation. Recent evidence indicates that follicular fluid contains endogenous modulators of cortisol metabolism by 11 beta HSD1, the concentrations of which are associated with the clinical outcome of assisted conception cycles and are altered in cystic ovarian disease. In conclusion, the two cloned isoforms of 11 beta HSD fulfil diverse roles in a wide range of reproductive processes from conception to lactation.