The development and the reactivity of fetoplacental circulation appear to be extremely well coordinated phenomena which follow a precise chronology and are regulated by numerous angiogenic and vasoactive factors. The endothelium of the villous vessels, due to its capacity to release vasodilatating substances, would appear to play a crucial role in the gradual decrease in the vascular resistance of the placenta, thereby enabling it to meet the growing needs of the fetus right up until the end of pregnancy. In the endothelium, as in the smooth muscle of the placental vessels, control of production and mechanisms of action of the signal modulators of placental tone--endothelin 1, nitric oxide, prostanoids, etc.--as well as their dependence on oxygen, are far from being entirely elucidated. The study of their interactions could contribute to better evaluating the relative importance of each of them in the particular balances which are established at different stages of normal pregnancy, as well as during pathological situations (pre-eclampsia, intrauterine growth retardation, etc.) when placental blood flow and fetal development are at risk of being compromised.