The effect upon steroidogenesis of adding various steroids produced by the placenta was studied in short term cultures of human fetal adrenal cells. The addition of high concentrations (10(3) ng/ml) of estrone or estriol inhibited the production of cortisol, but only the former elicited a parallel increase in dehydroepiandrosterone (DHA) production. Estradiol was effective in inhibiting delta-4-3-ketosteroid production at concentrations of 10-100 ng/ml, levels which approach those found in the fetal circulation, while DHA production was increased at concentrations of 1 microgram/ml. The addition of progesterone (4 microgram/ml) to the medium caused increased production of cortisol and corticosterone, but had no effect on DHA production. Pregnenolone (4 microgram/ml) increased the basal production of DHA and slightly impaired both basal and ACTH-stimulated aldosterone production, but had no effect on cortisol production. The data demonstrate that the many fetal and placental factors which have been studied to date, only ACTH and estrogens can interact to produce the characteristic fetal pattern of steroidogenesis. Preliminary studies indicate that this effect-stimulated aldosterone production, but had no effect on cortisol production. The data demonstrate that the many fetal and placental factors which have been studied to date, only ACTH and estrogens can interact to produce the characteristic fetal pattern of steroidogenesis. Preliminary studies indicate that this effect-stimulated aldosterone production, but had no effect on cortisol production. The data demonstrate that the many fetal and placental factors which have been studied to date, only ACTH and estrogens can interact to produce the characteristic fetal pattern of steroidogenesis. Preliminary studies indicate that this effect of estrogen is not influenced by other peptide hormones such as hCG, human prl, beta-lipotropin, corticotropin-like intermediate lobe peptide, or beta-endorphin. A revised model of the fetoplacental steroidogenic unit is presented which may explain both normal and fetal hyperplasia and postnatal involution of the adrenal cortex and the variations from this pattern seen in apituitary children.