Stabilised human serum 500 ml was infused intravenously over 90 minutes in 14 hypertensive women in late pregnancy, and the haemodynamic changes were investigated and compared with those in 7 similar women who were not treated. There was a significant mean increase of 1,85 l in plasma volume, a decrease in diastolic and systolic blood pressure, and an increase in central venous pressure (CVP), pulse pressure and pulse rate in the treated group at 2 hours but not in the control group. After 24 hours most of the observations were not significantly different from the pretreatment levels except the CVP and pulse rate measurements which were still significantly raised. The CVP measurements in the hypertensive women before treatment were relatively low compared with those reported in normal women in late pregnancy. It is suggested that there may be an under-filling of the circulation in pregnancy hypertension and that plasma volume expansion may have an important therapeutic effect by increasing cardiac output and renal and uterine blood flow.