Abnormal placentation is the likely cause of the slow fetal growth and the high levels of circulating lipid peroxides found in severe pre-eclampsia. These peroxides are probably responsible for the high thromboxane:prostacyclin ratio found in this disease and may participate in the endothelial cell damage which is its most notable feature. Selenium (Se), because of its role in glutathione peroxidase, is suggested to be an important component of the removal system for these damaging peroxides. Serum-Se concentrations have therefore been measured in 19 pairs of pre-eclamptic women and matched controls. Infant birth-weights were recorded. No significant difference was found in the concentrations of Se in pre-eclamptic and control groups. Serum Se was found to be low in both groups. Birthweights were significantly lower in the pre-eclamptic group. The interpretation of serum-Se measurements from the third trimester of a pre-eclamptic pregnancy is complicated by the reduced fetal growth and probable lower Se take-up by the fetus in such a pregnancy. The merits of alternative measurements, such as total intravascular Se, placental Se, or samples from an earlier stage of gestation, are discussed. The importance of factors other than Se to the activity of glutathione peroxidase, and of other antioxidants to pre-eclamptic, is stressed.