Plasma oestriol concentrations were studied in 48 women with prolonged pregnancies. In all, 177 studies were made from the 40th to 46th weeks. There was a significant fall in plasma concentration from the 40th to 46th weeks contrasting with the significant rise from the 24th to 42nd weeks of normal pregnancy. There was a correlation between the plasma concentration and urinary excretion of oestriol, as there is in normal pregnancy from the 24th to 42nd weeks, but there was a significant difference in the correlations (at these two stages) pointing to an increased renal clearance of oestriol during prolonged pregnancy. It is postulated that this phenomenon might depend on a proportionate increase in the production of the conjugate oestriol‐16‐glucosiduronate, which is more rapidly excreted by the kidneys than oestriol‐3‐sulphate, and which reflects a reduced metabolic activity. Urinary oestriol excretion, however, falls during prolonged pregnancy, despite the postulated increased clearance. It is concluded, therefore, that oestriol production, which reflects the function of the feto‐placental unit, is, in fact, reduced during prolonged pregnancy. Plasma oestriol studies would appear, therefore, to give a better physiological control of prolonged pregnancy than urinary oestriol studies.