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Cardiac hypertrophy was induced in rats by constructing an arteriovenous fistula. Heart weights approximately doubled in 7.5 weeks. The levels of activity, expressed per gram of heart, of a variety of enzymes of the mitochondrial respiratory chain and of the citric acid cycle, as well as the concentrations of cytochrome c and of mitochondrial protein, were the same in the hypertrophied and control hearts. Similarly, the capacity of whole heart homogenate to oxidize pyruvate was unaltered in hypertrophy.There was also no change in the levels of activity of cytochrome oxidase and malate dehydrogenase, or in the concentration of cytochrome c in early hypertrophy (3 to 10 days after construction of an A-V fistula). These results suggest that cardiac mitochondria increase in parallel with the other components of the myocardial cell.The levels of activity, per gram of heart, of creatine phosphokinase and adenylate kinase, and of the rate-limiting enzymes of glycolysis and glycogenolysis, were the same in the hypertrophied and the control hearts.The above findings suggest that the capacity, per gram of heart, for regenerating ATP, both aerobically and anaerobically, is unchanged in the nonfailing, hypertrophied heart. ADDITIONAL KEY WORDS cell fractions cell growth protein concentrations water contentFrom the Section of Cardiovascular Surgery,