SummaryLipoprotein lipase activity in lung and heart was studied in fetal (17-22 days of gestation) and newborn rats from the day of birth until 30 days of age. Enzyme activity in epididymal, omental, and parametrial adipose tissue was tested after 18 days of age. Blood triglyceride levels were measured at all ages from 17 daysin uterountil 30 days after birth.The developmental pattern of lipoprotein lipase differed markedly in lung and heart. Although lipoprotein lipase activity was 4 to 5 times higher hi adult rat heart than in lung (30-40 U/g in heart vs. 8-11 U/g in lung), the activity was almost completely absent from fetal heart and was very low during the first 3 days after birth. Lipoprotein lipase reached 60-70% of adult activity at 6 days and remained at that level until 19 days after birth; adult activity levels were reached at 24 days. In the lung, contrary to the heart, lipoprotein lipase activity was high in the fetus (84% of adult activity), decreased immediately after birth to 45% of adult activity at 2 days, and remained at that level up to 15 days after birth. Enzyme activity started to rise again at 15 days and reached adult levels at 21 days of age. Adipose tissue was present in trace amounts before the age of 2 weeks. In the three fat depots tested, lipoprotein lipase activity was 50% lower than in adults between 20 and 30 days after birth. Blood triglyceride levels increased 4- fold between 2 to 10 hr after birth and remained elevated during the first 3 days after birth.SpeculationThe triglyceridemia that starts in the immediate postnatal period and lasts for several days in the rat probably results from the combination of high fat intake and low clearing ability of the extrahepatic tissues.In addition to its role in clearing circulating triglyceride, lipoprotein lipase may play a role in the growth and maturation of individual organs. The present study shows that lipoprotein lipase activity is high in fetal lung during the period of marked surfactant synthesis and suggests that circulating triglyceride-fatty acids are used by the fetal lung for surfactant synthesis.