Morphometric data on left ventricular papillary muscle structures have been determined in tumor-induced malnutrition and related to the maximum activities of key enzymes for energy production in the whole myocardium. Adult, nongrowing mice with a syngeneic sarcoma were used to represent a condition of cancer associated host tissue wasting. Hearts from mice 11 days after tumor implantation showed atrophy and a significantly reduced amount of myofibrillar, soluble, and collagen proteins than hearts from control animals. The cross-sectional area of myocardial cells was 33% smaller in tumor-bearing mice (p less than 0.025), but the total number of capillaries and the residual interstitial volume were similar in the two groups. The total number of subcellular structures per cell, such as mitochondria, myofibrils, and myosin filaments per myofiber, were significantly lower in the tumor-bearing animals (p less than 0.025). Conversely, the proportion of myofibrils was higher (p less than 0.05) in tumor-bearing animals while the proportion of mitochondria was lower. Maximum activities (Vmax) of selected regulatory key enzymes for energy production (glycogenolytic, glycolytic, and mitochondrial) were not significantly altered in hearts from tumor-bearing mice. The results support the conclusion that myocardial functional capacity is better preserved than overall structural components would imply in tumor-host associated malnutrition, which is probably secondary to deprived food intake. Teleologically, this may be a means by which functional deterioration of the heart is minimized during the induction of malnutrition.