The effect of increasing tumor burden on host liver and skeletal muscle energy status was studied using P-31 nuclear magnetic resonance spectroscopy (NMR), in rats inoculated with a nonmetastasizing methylcholanthrene-induced sarcoma (TB), and compared to nontumor bearing (NTB) and pair-fed (PF) rats. During the 28-day study, serial measurements of body weight, food intake, and tumor volume were obtained. Using a 0.9-cm double-turn surface coil, weekly NMR measurements were obtained from liver and skeletal muscle. An increasing ratio of [Pi]/[ATP] was used as one measure of intracellular energy depletion. [Pi]/[ATP] in NTB rats remained constant over time at 0.78 +/- 0.10 in liver, and 0.30 +/- 0.10 in skeletal muscle. In TB rats, the [Pi]/[ATP] ratio increased significantly in liver (P = 0.00002) and skeletal muscle (P = 0.04) with increasing tumor burden. In PF rats, no significant change occurred in [Pi]/[ATP] in liver or skeletal muscle, indicating that declining food intake was not responsible for the change in [Pi]/[ATP] seen in TB rats. Surface-coil spectroscopy of liver and skeletal muscle permits serial measurement of visceral energy stores. Increasing tumor burden results in early, ongoing depletion of energy stores as reflected by increasing [Pi]/[ATP] in these organs.