To determine the ability of various tissues to incorporate essential fatty acids, rats maintained with a diet deficient in essential fatty acids for 25 weeks were transferred to a diet containing 10% corn oil. The fatty acid compositions of plasma, erythrocyte and liver lipids were determined at intervals over an 81-day period. Lipids from the deficient animals were low in linoleic and arachidonic acids and high in palmitoleic, oleic and eicosatrienoic acids. Supplemental feeding with corn oil resulted in a rapid increase in linoleic and arachidonic acids and decrease in palmitoleic, oleic and eicosatrienoic acids in the plasma and liver, although the latter acid remained above the level in the control animals for a longer period than palmitoleic and oleic acids. In the erythrocyte, linoleic acid was rapidly incorporated and oleic acid lost with equal facility. Incorporation of arachidonic acid, while initially rapid, proceeded throughout the 81 days of the experiment. Loss of eicosatrienoic acid was also a long-term process. The rate of incorporation of essential fatty acids by tissues from deficient rats was a function of each tissue and each fatty acid.