Alterations in the concentrations of the cholesterol and triglyceride moieties of lipoproteins separated by ultracentrifugation and precipitation methods were studied at frequent intervals throughout pregnancy and the puerperium in a group of 43 women. The plasma cholesterol concentration rose on the average by about 50 per cent, the major increase occurring in the second trimester. The plasma triglyceride concentration rose threefold, reaching its peak during the third trimester. All major lipoproteins participated in these changes: in very-low-density lipoproteins, both lipids rose in proportion to the ratio in nonpregnant women, but in low-density and high-density lipoproteins, the ratio of triglyceride to cholesterol rose. The triglyceride enrighment in low-density lipoproteins reflected the inclusion of intermediate-density lipoproteins (d 1.006 to 1.019). The occurrence of hypertension or pre-eclampsia led to a further increase in lipids in very-low-density lipoproteins. Hypercholesterolemia was greatest in women with pre-existing hypercholesterolemia, and women in the third pregnancy showed higher plasma cholesterol concentrations than women in the first pregnancy. Both cholesterol and triglyceride concentrations decreased significantly within 24 hours of delivery and this was reflected in all lipoproteins. However, while triglyceride levels continued to decrease rapidly returning to nonpregnant levels during the puerperium, cholesterol in low-density lipoprotein remained elevated for at least six to seven weeks post partum.