The microsomal fraction of the human fetal adrenal gland showed NADPH dependent enzyme activities of 17α-hydroxylase and C 17-C 20lyase. The apparent Michaelis constants ( K M ) of 17α-hydro xylase and C 17-C 20lyase for NADPH were 6 × 10 −8M and 3 × 10 −7M, respectively. The apparent k m value of 17α-hydroxylase for pregnenolone was 1.3 × 10 −8M and that of C 17-C 20lyase for 17-hydroxypregnenolone was 1.2 × 10 −7M. These enzymes were inhibited by most of the steroids produced in the feto-placental unit. 5-Pregnene-3β,20β-diol (apparent K i = 1.5 × 10 −8M), pregnenolone-sulphate (2.4 × 10 −8M), 5-pregnene-3β,20α-diol (3.1 × 10 −8M), 17-hydroxypregnenolone (6.1 × 10 −8M) and progesterone (8.1 × 10 −8M) inhibited the activity of 17α-hydroxylase toward pregnenolone. The following steroids were the inhibitors of C 17-C 20lyase toward 17-hydroxypregnenolone; 5-pregnene-3β, 20β-diol (apparent K i = 0.6 × 10 −7M), 5-pregnene-3β,20α-diol (1.7 × 10 −7M), progesterone (3.4 × 10 −7M), 17,20β-dihydroxy-4-pregnen-3-one (5.6 × 10 −7M), 17-hydroxyprogesterone (7.6 × 10 −7M), dehydroepiandrosterone (2.3 × 10 −6M), 5-androstene-3β,17β--diol (3.2 × 10 −6M), 16α-hydroxydehydroepiandrosterone (3.2 × 10 −6 M), testosterone (6.2 × 10 −6M) and estradiol-17β (9.2 × 10 −6M). The type of inhibition of these steroids appeared to be competitive.