Arterial hypertension was produced in 66 rats by means of constricting latex capsules about the kidneys, by the intramuscular implantation of desoxycorticosterone pellets, and by the subcutaneous injection of desoxycorticosterone. The most satisfactory method was the intramuscular implantation of desoxycorticosterone pellets. Myocardial hypertrophy was produced in 43 rats.Biochemical analysis of the ventricles of 80 control and 45 hypertrophied rats were performed for creatine phosphate, adenosine triphosphate, and free inorganic phosphate. Cardiac hypertrophy in rats was accompanied by a statistically significant decrease in the ventricular concentration of creatine phosphate, while levels of adenosine triphosphate and free inorganic phosphate were not significantly altered.It is suggested that congestive heart failure in hypertrophied hearts is related to a defect in the utilization of phosphate-bond energy by the hypertrophied myocardial fiber.