The triglyceride concentration increased gradually after the 10th day of gestation and it increased continuously until the time of delivery. Then it decreased immediately after birth. The phospholipid concentration also increased with gestational age, but cholesterol did not. The fatty acid components of fat emulsion (Intralipids) were labeled with carbon-14 and administrated into pregnant rats, then the fate of these fatty acids in blood was followed with time. The plasma half life was 74 minutes in pregnant rats and 58 minutes in non-pregnant rats. The disappearance was slower in the pregnant rat. This results agree with human studies. The uptake of radioactivity by the fetal liver was only 1/10 that of the mother. The radioactivity level in the expired 14CO2 was examined. 18% of radioactivity appeared in expired CO2 during 5 hours in pregnant rat. While, 30% was collected in non-pregnant animals, showing slower lipid metabolism during pregnancy.