Steroid biosynthesis in bovine fetal ovarian tissue was studied in vitro. Finely minced fetal ovaries (2.5 g) from animals more than 1 month before term were incubated with androstenedione-4-14C and progesterone-4-14C in separate flasks. The incubations were carried out for 4 hr at 37 C in phosphate buffer containing nicotmamide at pH 7.2. With progesterone as substrate, ATP, DPN, TPN and TPNH were added. When androstenedione was substrate, ATP and TPNH were added. Incubation with progesterone (29 μg) yielded several products. Five of these compounds were isolated and structure proof was effected using the carrier-dilution technique. These compounds and percentage conversions were: 17α,20α-dihydroxy- Δ4-pregnene-3-one (10.7 %), 17α-hydroxy- Δ4-pregnene-3-one (4.3 %), 16α-hydroxy- Δ4-pregnene-3-one (1.5%), testosterone (0.25%) and androstenedione (1.9%). A sixth compound has been tentatively identified as 20α-hydroxy- Δ4- pregnene-3-one (14.5%). After incubation with androstenedione (256 μg), testosterone (5%) was recovered and structure proof was established as above. The phenolic fraction was essentially devoid of radioactivity when progesterone was substrate, but with androstenedione a small fraction (less than 1 % of added radioactivity) with the Rf of estradiol-17β was present. Carrier estradiol-17β was added and structure proof was established by recrystallization to constant specific activity. Final conversion was 0.2%. Estrone and estriol could not be identified. These data demonstrate that the bovine fetal ovary is capable of effecting 17a-hydroxylation, 16α-hydroxylation, 20α-reduction, 17β-reduction, cleavage of the side chain and aromatization. They further suggest that the fetal ovary, like the fetal testis, may be active in utero. (Endocrinology74: 846, 1964)