Dietary fatty acids in the sn-2 position are preferentially absorbed as monoacylglycerols. To determine whether thy also have more important biologic effects, rats were fed for 2 and 4 mo a purified diet containing native palm oil, interesterified palm oil, native lard or interesterified lard. Interesterification that increased or decreased the level of fatty acids in position 2 depending on the fat, resulted in significant corresponding changes in the fecal excretion of saturated fatty acids. Fecal excretion of saturated fatty acids was associated with significant changes in some plsma fatty acids. Interesterification in lard resulted in significantly lower plasma triglycerides, and in palm oil, increased platelet aggregation induced by ADP. Lipemia, platelet aggregation and associated plasma fatty acids (palmitic, heneicosanoic and docosahexaenoic acids) were significantly affected only by dietary fatty acids at the sn-2 position. Even without changes in absorption, only linoleic acid in position 2 was correlated with the plasma concentration of the corresponding longer chain arachidonic acid. These results in rats confirm that the fatty acids (saturated and unsaturated) in position 2 of dietary triglycerides play a crucial role in the metabolism and biologic effects of these fatty acids.