Genistein, a plant-derived compound, is a phytoestrogen and has a quite well-documented effect on carbohydrate and lipid metabolism in animals. Data is lacking, however, about the influence of this compound when the level of endogenous oestrogens is high, for example in pregnancy. The aim of the performed experiment was to investigate the effect of dietary genistein (100 mg/kg of feed) on some blood hormone concentrations and metabolic parameters in pregnant rats. Two groups of pregnant Wistar rats were used: the animals in one group were fed a diet without genistein and in the other, a genistein-supplemented diet. Animals from both groups were sacrificed on day 13 or 18 of gestation or on the first day after delivery. Additionally, one group of control non-pregnant rats was fed a diet without genistein and used to compare with pregnant rats. The blood sera, livers and skeletal muscles of animals were collected and stored (-80 °C) until analysis. Genistein caused a substantial decrease of the serum leptin concentration on day 18 of gestation and of leptin and ACTH concentrations on the first day after delivery. The serum insulin, glucagon and corticosterone concentrations remained unaffected by genistein. The tested compound did not influence serum glucose, triglycerides, free fatty acids, cholesterol or liver and muscle glycogen, triglycerides and cholesterol. These results suggest that genistein, despite its well-described oestrogenic potency, can not manifest its action during pregnancy because of high levels of endogenous oestrogens