The heparin-releasable liver lipase of the rat has a modest phospholipase A1 activity when dipalmitoylphosphatidylcholine is used as the substrate. The activity is more than doubled when dioleoyl phosphatidylcholine or soybean phosphatidylcholine (mainly containing esterified linoleic acid) are used instead. The hydrolase and transacylase activities of the enzyme with monoacylglycerol as the substrate also increase when unsaturated acylglycerols are used instead of saturated monoacylglycerols. The ratio of the rates of hydrolysis to transacylation increases when more saturated monoacylglycerols are used or when the availability of unsaturated monoacylglycerols (by the addition of albumin) to the enzyme is diminished. Therefore, in vivo the enzyme may be expected to have mainly hydrolytic activities, acting upon phospholipids and monoacylglycerols of lipoprotein particles and on monoacylglycerols bound to albumin. The higher activity on unsaturated fatty acid derivatives may contribute to the hypolipidemic effect of dietary (poly)unsaturated fatty acids.