Variations of the plasma and erythrocyte lipid composition were studied in 110 clinically normal pregnant women at different gestational ages; a longitudinal study, carried out in 10 of these women, confirms the results of the cross-sectional study. The main changes observed in the plasma are a rise of the levels of triglycerides, phosphatidyl-cholines and to a lower extent of sphingomyelins; these increases are significantly correlated with the gestational age. The RBC lipid composition is faintly modified throughout pregnancy; the most important changes are observed close to term: after 35 weeks of gestation, the concentration of sphingomyelins, phosphatidyl-cholines and phosphatidyl-ethanolamines are higher and that of the cholesterol/phospholipids is lower than at the beginning of pregnancy. Modifications of some erythrocyte physical properties occurring in the course of gestation cannot mainly be attributed to variations of the membrane lipid composition.