SUMMARY Fifty-two normal human placentae from the 6th to the 42nd week of pregnancy were surveyed for 3α-, 3β-, 6β-, 11α-, 11β-, 12α-, 16α-, 16β-, 17α-, 17β-, 20β-, 21- and 24-hydroxysteroid dehydrogenase (HSD) activities. Strong NAD-linked 3β-, 16β-, and 17β-HSD activities and moderate NADP-linked 3β-, 16β- and 17β-HSD activities were found in the villous trophoblast at all ages. Weak 3α- and weak or trace 6β- and 11β-HSD activities were found in the trophoblast of some placentae using NAD as cofactor, with no obvious age distribution. Moderate 17β-HSD activity, NAD-linked only, was found in the villous stroma and vasculature.