It has been suggested that fetal adrenal steroids affect the secretion of progesterone by the human trophoblast and decrease the progesterone/estrogen secretory ratio at the time of parturition. In the present study, cultured human choriocarcinoma JEG-3 cells were used as an experimental model in order to examine the effect of dehydroepiandrosterone sulfate on both de novo and low-density lipoprotein-stimulated progesterone secretion. In serum-free and cholesterol-free Dulbecco's modified Eagle's medium, JEG-3 cell cultures demonstrated a significant secretion of both pregnenolone and progesterone. A 200% to 300% increase in pregnenolone and progesterone secretion was achieved by physiologic low-density lipoprotein concentrations added to Dulbecco's modified Eagle's medium while androgens and estrogens remained undetectable. Dehydroepiandrosterone sulfate added to Dulbecco's modified Eagle's medium was actively converted into C-19 and C-18 steroids but had no significant effect (up to 20 micrograms/ml) on basal (de novo) progesterone secretion. In contrast, the addition of dehydroepiandrosterone sulfate (1 to 5 micrograms/ml) to cultures grown in Dulbecco's modified Eagle's medium plus low-density lipoprotein induced a dose-related inhibition of progesterone and a return of that secretion to basal levels.