The effects of oxytocin on fetal and placental growth and on maternal weight gain and accumulation of body fat were studied inad libitum-fed and food-restricted (receiving 70% of the food intake of thead libitum-fed group) pregnant rats. Further, a possible role of the IGF axis in mediating oxytocin-induced changes was assessed. Pregnant rats were injected subcutaneously once a day during gestational d 1-5 with saline or oxytocin (1 mg/kg).Ad libitum-fed oxytocin-treated pregnant rats had higher circulating levels of IGF-I, larger placentas, fetuses, and newborn pups and contained less body fat at the end of pregnancy. In food-restricted dams, oxytocin-treatment had no effect on fetal and placental growth. Additionally, food restriction attenuated the normal increase in IGF binding protein-3 protease proteolysis during pregnancy. The results show that oxytocin may affect maternal adaptations to pregnancy and stimulate fetal growth. We suggest that this effect may be mediated by increased IGF-I inad libitum-fed animals, whereas food restriction may block this effect by resulting in low levels of circulating IGF-I and by attenuating the pregnancy-associated increase in IGF binding protein-3 protease activity and, thereby, further compromise IGF bioavailability.