Abstract Malnutrition has been associated with changes in cardiac metabolism and performance. We have previously reported a diabetic-type cardiomyopathy associated with chronic food restriction and weight loss [ 28]. Because the creatine-phosphocreatine-creatine kinase system is important in the contractile process, we studied the components of this system in rats fed a food-restricted diet (33% of control animal intake). After 4 weeks of food restriction, total creatine kinase (CK) activities were reduced by 28% in ventricles and by 38% in atria. The CK isoenzymes in the heart were not equally affected. The BB isoenzyme was decreased by 77% and 78%, the MB isoenzyme by 45% and 43%, the MM isoenzyme by 22% and 19% and CK mito by 16% and 15% in ventricles and atria, respectively. In contrast, brain CK activity which is predominately the BB isoenzyme, was slightly higher in the food-restricted than in control rats. Further studies on ventricular tissue from food-restricted rats revealed a 27% decline in myofibrillar CR activity and a 58% decline in myofibrillar ATPase activity. Phosphocreatine and creatine concentrations were not changed by food restriction, however ATP was decreased by 23% in ventricles from rats on the restricted diet. Cardiac mitochondrial oxidative phosphorylation was also impaired. State 3 respiration with α-ketoglutarate was reduced 20% in the food-restricted heart. These changes are compared to those which we previously observed in the diabetic rat heart [ 29] and the significance of these findings is discussed.