Rats were fed rapeseed oil rich in eicosenoic (20:1) and docosenoic (22:1) acids for 7 days, and the fatty acid composition of the lipid classes of serum and serum lipoproteins was determined. Concentrations of 20:1 and 22:1 acids in the lipid classes were variable, especially among lipoproteins, and were a direct function of the alimentary state of the animal. The results suggest differences in the incorporation of the above acids among the major lipoprotein types and various lipid classes within a given lipoprotein type. The quick partial disappearance of very low density lipoproteins (VLDL) and of low density lipoproteins (LDL) containing 20:1 and 22:1 acids upon starvation and the preferential incorporation of these acids in the triacylglycerols of high density lipoproteins (HDL) are discussed.