The type 2 isoform of 11 β-hydroxysteroid dehydrogenase (11β-HSD2), which catalyzes the conversion of cortisol to hormonally inactive cortisone in man, is principally expressed in the placenta and mineralocorticoid target tissues, kidney and colon. To date, few studies have addressed the regulation of this novel 11β-HSD2 isoform. We have characterized the nature and regulation of the 11β-HSD activity expressed in a human cytotrophoblastic cell line, the JEG-3 choriocarcinoma cell. The 11β-HSD activity in JEG-3 cell homogenates required NAD+ as cofactor with NADP ineffective and demonstrated a high affinity for cortisol (apparent K m 31 nm). Incubation of JEG-3 cells with forskolin and dibutyryl cyclic AMP increased 11β-HSD2 activity several-fold in a time-dependent manner, while treatment with phorbol ester had little, if any, effect on 11β-HSD2 activity. Northern blot analysis of RNA isolated from JEG-3 cells after these treatments demonstrated a marked increase in a 1·9 kb 11β-HSD2 mRNA species in cells treated with forskolin for 24 h. We conclude that 11β-HSD2 is regulated by activation of the protein kinase A pathway, but not the protein kinase C pathway in human choriocarcinoma cells, and that this regulation occurs at a pretranslational level. JEG-3 cells provide an excellent model for further studies on the regulation of 11β-HSD2 gene expression in human trophoblast tissue.