Genistein is a phytoestrogen found in soy beans. Phytoestrogens have been reported to cause reproductive problems in sheep and rats. This research was conducted to determine the effects of genistein fed to rhesus monkeys during pregnancy, with specific interest on fetal growth and steroidogenesis in the maternal-fetoplacental unit. Two groups of five monkeys each were selected in early stages of pregnancy. One group was administered genistein in a fruit treat each weekday until Cesarean section 10 days prior to term. The second, control group, received fruit treats without genistein. Maternal blood samples were collected on Tuesday and Friday of each week. At delivery, samples were collected from the maternal peripheral circulation, uterine veins, uterine-ovarian veins, and the fetal heart. Comparisons between control and genistein-treated monkeys revealed no differences in the maternal weight gained during pregnancy, or in fetal weights or placental weights at delivery. Serum was assayed by radioimmunoassay (RIA) for estradiol, progesterone, dehydroepiandrosterone sulfate (DHEA-S), and estrone. No significant differences (P > 0.05) were noted in progesterone or DHEA-S levels at delivery or during the pregnancy; however, estradiol levels were higher (P < 0.05) in the four areas studied at delivery and in the maternal blood with advancing gestation. Additionally, estrone levels tended to increase more rapidly (P = 0. 057) in the maternal blood of monkeys receiving genistein than in untreated controls, suggesting that genistein may stimulate the deconjugation of estrone in the gut, thus allowing its reabsorption into the peripheral circulation and conversion to estradiol.