Currently it is difficult to predict the efficacy of any therapeutic modality in individual patients. If it could be shown that successful therapy causes some chemical alteration in the tumor before gross alteration in size becomes radiologically visible, the therapeutic regimen could potentially be modified, sparing the patients longer trials of ineffective therapy. We used proton nuclear magnetic resonance spectroscopy to detect the presence of simple metabolites (such as lactic acid, creatine/phosphocreatine, N-acetyl aspartate, and the "choline" pool) in extracts of a human glioma grown subcutaneously in athymic ("nu/nu") mice. By comparing the tumor spectra obtained from untreated mice with tumor spectra from mice treated with chemotherapy or irradiation, we have shown a significant decrease in the lactate/creatine and lactate/choline values in tumors of similar size following treatment. Such information could prove valuable as a means of monitoring tumor therapy when obtained noninvasively from spatially localized in vivo spectra.