Placental 11beta-hydroxysteroid dehydrogenase (11beta-HSD) regulates transplacental passage of maternal glucocorticoids to the fetus and is thus a key determinant of fetal glucocorticoid levels. It has also been proposed that placental 11beta-HSD expression may influence local glucocorticoid actions by regulating access of corticosterone to the glucocorticoid receptor (GR) or mineralocorticoid receptor (MR). Therefore, the present study used a rat model to assess whether the GR or MR are coexpressed with the two forms of 11beta-HSD (types 1 and 2) in the placental labyrinth zone, the major site of maternal-fetal transfer, and in the basal zone, the primary site of placental hormone synthesis. In situ hybridization analysis was used to assess messenger RNA (mRNA) expression for the GR, MR, 11beta-HSD-1, and 11beta-HSD-2 in the two placental zones on days 16, 19 and 22 of pregnancy (term = day 23). Whereas expression of the GR appeared relatively unchanged in both zones at these three stages of pregnancy, that of 11beta-HSD-1 clearly increased in the labyrinth zone but fell in basal zone, whereas the opposite pattern of expression was observed for 11beta-HSD-2. MR expression was not detected at any stage. The pattern of placental 11beta-HSD-2 mRNA expression over days 16, 19, and 22 of pregnancy was paralleled by changes in 11beta-HSD-2-specific bioactivity, but despite clear expression of 11beta-HSD-1 mRNA, no bioactivity attributable to this enzyme was measurable in either placental zone. To assess the role of fetal adrenal maturation on these changes in 11beta-HSD, two experimental models, maternal adrenalectomy and fetectomy, were employed. Maternal adrenalectomy on day 13 advanced maturation of the fetal adrenal cortex but had no effect on 11beta-HSD-2 bioactivity in either of the placental zones at day 19. Placental 11beta-HSD-2 bioactivity on day 22 was also unaffected by fetectomy 3 or 6 days earlier. In conclusion, the consistent expression of the GR in the two placental zones late in pregnancy suggests that concomitant and marked changes in 11beta-HSD-1 and 11beta-HSD-2 expression could have a major influence on glucocorticoid action in the placenta at this time. Moreover, the changes in 11beta-HSD expression appear to be unrelated to development of the fetal adrenal cortex and are likely to reduce the placental glucocorticoid barrier near the end of pregnancy.