Duplicate experiments were conducted to compare energy utilization, growth, cardiac mitochondrial oxidative phosphoryl,tion, and mitochondrial membrane fatty acid composition of chicks fed diets containing 20 parts of high erucic acid rapeseed oil (HER), low erucic acid rapeseed oil (LER) or sunflower seed oil (SFO) for 24 days. Chicks fed diets containing HER deposited less fat and utilized energy less efficiently (kcal gained/kcal consumed) than chicks fed diets containing either LER or SFO. Energetic efficiency and fat deposition of chicks pair-fed diets containing LER were significantly lower than for chicks fed diets containing SFO. Cardiac mitochondria isolated from chicks fed diets containing either HER or LER for 24 days had significantly reduced ADP/O ratios and reduced rates of ATP synthesis utilizing pyruvate and malate as the respiratory substrates when compared with mitochondria isolated from chicks fed SFO. Diet induced transitions in fatty acid composition of cardiac mitochondrial membranes were also observed. The composition of fat ingested affected the fatty acid composition of mitochondrial diphosphatidyl glycerol more than the fatty acid composition of phosphatidyl choline or phosphatidyl ethanolamine. The linoleic acid content of mitochondrial diphosphatidyl glycerol was lower and the gadoleic acid and erucic acid content higher for chicks fed diets containing rapeseed oils than for chicks fed SFO containing diets. These studies indicate that a complex dynamic mechanism exists associating dietary fat with mitochondrial structural-functional changes and energetic efficiency in the growing chick.