ABSTRACT.Postheparin plasma lipoprotein lipase and hepatic lipase activities of 11 preterm infants aged 1 to 4 days were measured 15 min after a heparin bolus of 100 IU/kg and during an exchange transfusion performed with fresh heparinized blood. Each infant had a birth weight (range, 1210–3490 g) appropriate for gestational age (range, 28–36 (wk). Eight of the infants (group 1) were in good clinical condition while three (group 2) suffered from septic shock. After the heparin bolus and during the exchange transfusion, lipoprotein and hepatic lipase activities in group 1 were higher than in term infants. In group 2, both lipase activities were extremely low after the heparin bolus but increased approaching the activities of group 1 during the exchange transfusion. Clearance of fat from the circulation is slower in preterm than term infants. This has been attributed to low lipoprotein lipase activity. Our results, however, indicate that lipoprotein lipase is not the reason for slow clearance of fat from the circulation in preterm infants except in septic shock.