The nature of the lipases released into the circulation during low level continuous infusion of heparin (1 unit/ml total parenteral nutrition) and after bolus heparin injection (10 units/kg) was investigated in a group of 11 low birth weight infants (gestational age 27–34 wk, and postnatal age of 7–26 days) receiving total parenteral nutrition with Intralipid (0.5 g/kg). Hepatic lipase and extrahepatic lipoprotein lipase were differentiated with the aid of an antibody specific for human hepatic lipase. The data show that continuous low level heparin infusion leads to a constant baseline postheparin lipolytic activity of 0.77 ± 0.18 µmol free fatty acids released per milliliter serum per hour. Bolus heparin injection leads to peak lipolytic activity levels of 3.77 ± 0.46 µmol free fatty acids per milliliter serum per hour, 10 min after injection. About two-thirds of the total postheparin lipolytic activity was of the hepatic type during low level continuous infusion or after bolus injection of heparin.