Quartered placentae from 12- and 15-day pregnant hamsters were incubated with 14C labelled pregnenolone and progesterone and products of their conversion, were identified by chromatographic and isotope dilution methods. Pregnenolone was converted to progesterone, 7 alpha-hydroxypregnenolone, 7 alpha-hydroxyprogesterone, 3 alpha--hydroxy-5 beta-pregnan-20-one, 3 beta-hydroxy-5 alpha-pregnan-20-one, 3 alpha-hydroxy-5 alpha-pregnan-20-one, 5 beta-pregnane-3,20-dione and 5 alpha-pregnane-3,20-dione. Except for 7 alpha-hydroxypregnenolone, the same metabolites were identified in incubates of the placental tissue with progesterone. Thus, the activity of delta 5-3 beta-hydroxysteroid dehydrogenase and delta 5 leads to 4 isomerase, 7 alpha-hydroxylase, 3 beta- and 3 alpha-hydroxysteroid dehydrogenase, delta 4-5 beta- and delta 4-5 alpha-reductase enzyme systems was shown in hamster placenta. The formation of androgens from pregnenolone, progesterone and their 17-hydroxy-derivatives was not observed. There was also no evidence for the formation of estrogens from the above C-21 steroid precursors.