Several structures in the myocardial cell are ultimately involved in the control of tension generation and relaxation in the heart. The sarcolemmal membrane, the sarcoplasmic reticulum, the mitochondria, and the contractile proteins are the principal subcellular organelles involved in contraction-relaxation in the normally functioning heart. The observation that contractile protein adenosine triphosphate activity could be closely correlated with cardiac muscle function has allowed researchers to obtain a biochemical marker of contractile protein function which also has important relevance to muscle performance. The presence of lesions in other subcellular organelles may also explain several abnormalities associated with the diabetic cardiomyopathy. Many of the lesions are dependent upon the duration of diabetes and are insulin-reversible, again suggesting that this subcellular dysfunction is closely associated with the diabetic condition. Purified myofibrillar fractions isolated from hearts of streptozotocin-induced, chronically diabetic animals exhibited depressed enzymatic activities in comparison to those observed in control preparations.