Abstract Progesterone is the main steroid produced by bovine blastocysts collected at days 13–15 and placed in TCM-199 supplemented with 5% calf serum. PGF and PGE; were increased ( P< 0.05) in the cultured blastocysts. It is suggested that PG synthetase and enzymes capable of synthesizing progesterone and possibly estradiol are present in these early bovine blastocysts. The source of the steroids produced by the blastocyst appear to be trophoblast. At day 28–30 of fetal life, a new source of steroid production appears in the form of the fetal gonads. The unditferentiated gonad at 30–40 days, was shown to produce progesterone but not testosterone or estrogen, regardless of the sex of the fetus, and did not respond to LH. Between 95 and 110 days the amount of testosterone produced per mg declined from 1703 ± 391 to 37 ± pg/mg gonad/24 h. Fetal testes from fetuses of 3.3 to 20 cm CRL were responsive to LH. Similar to the testes, the fetal ovary also showed a marked initial rise in steroidogenesis at the time of gonadal differentiation. However, the principal product of the ovary was 17β-estradiol which was produced in ng amounts. The initial production of 2674 ± 512 pg/mg gonad/24 h incubation at 3.3–4.5 cm dropped rapidly to 286 ± 176 pg/mg gonad/24 h at 7.8–8.5 cm CRL. Furthermore, 17β-estradiol production was undetectable in all older fetuses, while testosterone was readily detectable in testes of the same size. The mean values of 17β-estradiol secreted by the cultured ovaries from fetuses of 3.4 to 8.1 cm CRL was significantly enhanced by the presence of bovine LH. In the presence of testosterone the mean value of 17β-estradiol secreted by the incubated ovaries from fetuses of 5–8 cm and of 10–20 cm CRL was significantly increased from 1.5 ± 0.4 to 6.5 ± 0.7 ng and from ND to 2.2 ± 0.4 ng, respectively. In contrast, neither progesterone, 17α-hydroxyprogesterone, nor DHT affected 17β-estradiol secretion in ovaries of the older group. 8-Br-cAMP added to the culture medium significantly stimulated both 17β-estradiol and testosterone production by the cultured ovaries from fetuses of 5–8 cm CRL. However, when the refractory ovaries from fetuses of 10–20 cm CRL were examined, only progesterone secretion was significantly enhanced. The role of estradiol secretion by the early fetal ovaries is not clear. The high levels of testosterone produced by testes and 17β-estradiol by ovaries in 45 ± 3-day gonads suggest that bovine embryonic gonads are not only morphologically, but also functionally differentiated by this time.