In an attempt to clarify the mechanism of lipid metabolism during pregnancy, alpha 1-acid glycoprotein (alpha 1-AG) was analyzed in normal and diabetic pregnant women. Seventy-two determinations of serum alpha 1-AG levels were performed in 18 diabetic pregnant women and 82 determinations in 82 normal pregnant women in all three trimesters and within 14 days postpartum. Serum alpha 1-AG levels in both normal and diabetic pregnant women decreased throughout pregnancy and rapidly increased postpartum. In all gestational stages, the serum alpha 1-AG levels were lower in diabetic women than in normal women, but the differences were not significant. No significant correlation was obtained between serum alpha 1-AG and hemoglobin A1 (HbA1) in diabetic patients. On the contrary, the serum triglyceride levels increased during pregnancy and decreased postpartum in both groups of subjects. These findings suggest that serum alpha 1-AG plays an important role in the activation of lipoprotein lipase during pregnancy.