The in vivo and in vitro effects of heparin on the lipolytic enzyme activities of normal rabbit serum and tissues were investigated. An acute intravenous injection of heparin produced the following changes in enzyme activities: in serum, phospholipase and lipase were increased, cholesterol esteresa unchanged;in tissues phospholipase was increased, lipase was increased in heart and liver but decreased in aorta, cholesterol esterase was increased in heart but decreased in liver and aorta. In vitro phospholipase and lipase were increased by low concentrations of heparin, but decreased by high concentrations. Cholesterol esterase was decreased irrespective of heparin concentration. It is suggested that heparin may affect lipolytic enzyme activities by influencing the ionic state of the enzyme and substrate.