The progressive rise in pregnancy of oestriol values and their significance in assessing fetal viability is well known, although the exact function of this increase is not understood. In considering the problem it was thought that further information of oestriol levels in mother, fetus and newborn could prove of value. The mean cord plasma oestriol was lower in small-for-dates than in normal cases, but in both groups it was ten times higher than in the relative maternal peripheral veins. In pooled capillary fetal scalp samples the mean oestriol was slightly higher in normal compared with small-for-dates cases. The use of gas chromatography--mass spectrometry was investigated and shown to give an accurate measurement of plasma oestriol levels. The expense of this procedure may be offset by its ability to determine simultaneously the nature and amount of other steroids present.