11 beta-hydroxysteroid dehydrogenase (11 beta-HSD) catalyzes the conversion of active cortisol to inactive cortisone, and regulates the access of cortisol to both the mineralocorticoid and glucocorticoid receptors. Two isoforms of 11 beta-HSD have been described, the cloned "type 1" NADP(H)-dependent dehydrogenase/oxo-reductase and a high affinity NAD-dependent dehydrogenase (type 2). In the fetus, 11 beta-HSD activity may serve to protect developing tissues from cortisol excess or may modulate the permissive actions of glucocorticoids. We have studied 11 beta-HSD activity and mRNA levels in human mid-gestational fetal tissues. Tissue homogenates were incubated with either 0.1 mumol/L cortisol and 400 mumol/L NAD, 2.5 mumol/L cortisol and 400 mumol/L NADP, or 0.1 mumol/L cortisone wither either 400 mumol/L NADPH or NADH. No activity (< 2.5% conversion) was observed in fetal tissues using either cortisone or 2.5 mumol/L cortisol as a substrate. 11-oxo-reductase activity was observed in maternally-derived decidua. In keeping with these activity studies, northern blot analysis of fetal tissue RNA and PCR-reverse transcriptase of type 1 11 beta-HSD mRNA indicated 11 beta-HSD mRNA in decidua, but failed to detect any type 1 11 beta-HSD mRNA transcripts in fetal tissues. In contrast when 0.1 mumol/L cortisol was used as a substrate in the presence of NAD, 11 beta-HSD activity was ubiquitous with highest levels seen in the kidney (131 +/- 16 (SE) pmoles cortisone formed/h/mg.protein) > lung > gonad > liver > colon. 11 beta-HSD activity in fetal tissues is mediated by the type 2, high affinity, isoform. The widespread distribution of this novel isoform suggests that it may play an important role in fetal development. Type 1 11 beta-HSD mRNA and activity are absent in mid-gestational fetal tissues, but present in maternally-derived decidua, suggesting that its ontogeny is a late-gestational of post-natal event.