Serum and lipoprotein (LP) lipids were measured in fetal pigs and their dams during gestation. In the 2nd trimester (70 days of gestation), fetal lipids resembled those of their dams. Early in the 3rd trimester (90 days) total serum concentrations of cholesterol (CH), triglyceride (TG) and free fatty acids (FFA) were lower in fetuses than in sows (p less than 0.01), but the decreases were not expressed uniformly among the LP classes. By the middle of the 3rd trimester (100 days) fetal serum lipids had declined further, and the distribution of TG (but not CH) between the LP fractions was significantly different from the distribution in sows (p less than 0.01). Sow serum CH, TG and FFA varied, but did not change significantly from days 70 to 100 of gestation; there was a tendency for sow serum lipids to increase, particularly FFA. Distributions of TG and CH in sow LP were unchanged by pregnancy. Results suggest that independent fetal regulation of lipid metabolism develops during the last half of gestation, and that lipid metabolism (especially TG) may be altered during fetal life. Changes in sow lipids during gestation were much less pronounced than in humans, but fetal patterns were similar.