Effects of serum from pregnant women on basal and FSH (or cAMP) stimulated aromatase activity of immature rat Sertoli cells in primary culture were studied. Pregnancy serum caused a dose-dependent stimulation of Sertoli cell aromatase activity and the response curves were parallel to those obtained with human FSH. This stimulatory (FSH-like) activity increased progressively during pregnancy, with a sharp drop immediately after delivery. However, the FSH-like bioactivity was not associated with immunoreactive FSH when a specific radioimmunoassay was employed. On the other hand, serum from pregnant women caused a dose-dependent inhibition of FSH and dibutyryl-cAMP-stimulated aromatase activity. These data suggest that human pregnancy serum contains factor(s) which may stimulate basal aromatase activity of Sertoli cells and may inhibit FSH-induced aromatase activity. These factors, most probably of placental origin, may play a role in the regulation of estrogen production during gestation.