The Authors studied the levels of Estetrol (15 alpha-hydroxyestriol) in the amniotic fluid, in maternal and foetal plasma, by the RIA method, in near-term pregnancies. Higher concentrations of this steroid were found in the foetal plasma and in amniotic fluid than in the maternal plasma. These data, even though of little clinical importance, confirm the foetal origin of this compound and suggest further studies, especially in the amniotic compartment.