The human placenta contains two types of 11beta-hydroxysteroid dehydrogenase (11beta-HSD). The exclusive oxidase 11beta-HSD2 has been suggested to protect the fetus from high levels of maternal glucocorticoids by converting cortisol to inactive cortisone. Perfused term human placenta was used to examine the activity of the oxoreductase 11beta-HSD1 and to determine the regulation of cortisol effects on placental vascular tone and corticotropin-releasing hormone (CRH) output by 11beta-HSD. Radioimmunoassay showed that there was substantial cortisol (295+/-57 nM) detected in the fetal vein upon perfusion of cortisol (2 microM; perfusion rate, 12 ml/min) into the maternal intervillous space. Output of cortisol increased to 559+/-22 nM on the fetal side (P<0.05) with concurrent perfusion of carbenoxolone (CBX; 1 microM), a non-specific 11beta-HSD inhibitor. Cortisol formation increased in a dose-dependent manner with infusion of cortisone (0.1-2 microM) into the maternal intervillous space reaching 15 and 23 nM in fetal and maternal venous outflows respectively at 2 microM cortisone perfusion. There was no significant effect of cortisol either alone or in combination with CBX on the fetal arterial perfusion pressure, but cortisol perfusion increased CRH output into the fetal vein. It is concluded that activities of both 11beta-HSD1 and -2 are demonstrable in perfused human placenta in vitro, and these enzymes affect transplacental glucocorticoid transfer. These activities may provide a precise mechanism to control the passage of maternal glucocorticoids to the fetal circulation, and to regulate glucocorticoid effects within the placenta.