The conversion of radiolabeled androgen to estrone and 17 beta-estradiol was assessed in tissues of human embryos that varied from phenotypically indifferent stages (1-3 cm crownrump length) to midgestation (15. 1-20 cm crownrump length). Significant rates of estrogen synthesis were demonstrated only in ovaries, liver, and brain. Estrogen synthesis was undetectable in gonads from 1-3 cm fetuses, but by the 3.1-5-cm stage it had reached an average rate of 1.9 pmol . h-1 . mg protein -1 in ovaries and remained at this level of activity through the latest stages examined. Estrogen formation was undetectable in testes at all stages examined, but the time of appearance of the capacity to form estrogens in the fetal ovary is similar to the onset of the capacity of the fetal testis to synthesize testosterone. The capacity of the fetal ovary to form estrogen develops before histological differentiation of the tissue.