In a comprehensive study the concentrations of plasma lipids and lipo- and apolipoproteins were measured in 24 nonpregnant women (control) and longitudinally in 42 women throughout gestation and postpartum. The results were correlated with hCG, 17β-estradiol (E2), progesterone (PG), human placental lactogen (hPL), and insulin levels by time series analysis. Insulin concentrations were constant until week 25 and increased thereafter. Plasma E2, PG, and hPL as well as plasma lipid levels rose continuously during gestation. Apolipoproteins AI, All, and B concentrations increased until weeks 25, 28, and 32, respectively, and remained constant until term. Low density lipoprotein cholesterol reached maximum levels at week 36. High density lipoprotein cholesterol exhibited a triphasic behavior, with maximum levels at week 25, a fall until week 32, and maintenance of the level until term. Time series analysis revealed positive correlations with E2, PG, and hPL. These results provide evidence that apoprotein concentrations undergo pronounced serial changes during gestation, which in part might be due to the effect of E2. Furthermore, the importance of hPL as a determinant of the plasma levels of total and free cholesterol, triglycerides, and phospholipids is now documented.