Because previous results had shown that experimental heart failure was not caused by reduction of oxidable substrate and high-energy phosphate content in myocardial tissue, further experiments have been performed in order to see if any alteration in subcellular membranes, which are strictly involved in the excitation-contraction coupling mechanism, could be detected in failing heart. Experimental cardiac failure was realized by imposing a high load on the left ventricle of the rat heart-lung preparation. The determination of mitochondrial and microsomal phospholipid classes has shown: (1) no change in mitochondrial phospholipids and (2) a 20% reduction in phosphatidylcholine, 35% in phosphatidylserine and 34% in sphingomyelin in the microsomal fraction from failed heart.