Longitudinal physiologic changes in blood viscosity parameters were studied in 9 women with uneventful pregnancies. After an initial decrease in blood viscosity, an increase towards the end of pregnancy was found.The oxygen-transport capacity remained quite constant during pregnancy with a small rise at gestational week 32.A cross-sectional study was performed in 102 pregnant women at 28 weeks gestation. Chronic dietary sodium restriction did not influence blood viscosity significantly. No significant relationships were found between hemorheologic parameters and parity, prematurity and fetal growth retardation. Higher blood viscosity was found in women who became hypertensive in pregnancy, even before the onset of the disease.