After 1 week, the level of myocardial fatty acids was 4 times greater in young rats fed high erucic rapeseed oil than in those fed oliver oil. The proportion of erucic acid was 5.6% in the mitochondrial fraction, 15.1% in the microsomal fraction, and 34.8% in the floating fat fraction. This incorporation of erucic acid into triglycerides of the floating fat was evidence of esterification. The changes in the mitochondrial lipids did not alter the content of adenine nucleotides of the myocardium nor its apparent capacity to oxidize substrates.