The purpose of this study was to determine the effects of the 7288ctc Morris hepatoma on heart size and performance. Hearts from tumor bearing and control animals were perfused in the working configuration one to three weeks post implantation. As tumor growth progressed there was an inverse linear relationship between tumor size and heart weight. When intrinsic heart work (defined as the product of the cardiac output and peak systolic pressure) was assessed in vitro over a range of physiologic preloads, significant differences were found between tumor-bearing and non-tumor-bearing animals. The hearts from the tumor-bearing animals (tumor weight 10-20 grams) developed only 76% of the heart work of control animals at maximal left atrial filling pressure (25 cm H2O). Hearts excised from rats with tumor masses from 30-65 grams developed 43% of the myocardial work as controls at the same (25 cm) preload. At the time of sacrifice resting blood pressures, blood glucose, insulin, and tumor necrosis factor (TNF) levels were similar in both groups. Blood from tumor bearing animals were negative for both aerobic and anaerobic bacteria. The data suggests that the decrements in cardiac size and performance are independent of several vectors known to influence heart size and performance and occur in the absence of detectable bacteremia.