The enzyme 11 beta-hydroxysteroid dehydrogenase (11 beta-HSD) catalyses the conversion of physiological glucocorticoids to inactive products, thus modifying the access of glucocorticoids to glucocorticoid and mineralocorticoid receptors. Glucocorticoids may affect ovarian function both indirectly and via binding to ovarian receptors. We have demonstrated 11 beta-HSD bioactivity and mRNA expression in rat ovary in vitro. The enzyme was localized to oocytes and luteal bodies immunohistochemically using two antibodies raised against purified rat liver 11 beta-HSD. These data are supported by in-situ hybridization studies, which also localized 11 beta-HSD mRNA expression to oocytes and luteal bodies. The results suggest that 11 beta-HSD may modulate the effects of glucocorticoid on ovarian function.