The authors have used phosphorus magnetic resonance spectroscopy to monitor pH changes in malignant gliomas following treatment with intravenous and intra-arterial 1, 3-bis-(2-chloroethyl)-l-nitrosourea (BCNU). Image-guided, localized phosphorus spectra of human gliomas in situ were obtained using a 1.5-T whole body combined imaging and spectroscopy system. Initial intravenous BCNU treatment was followed by a transient decrease of tumor intracellular pH by 0.15 ± 0.03 pH units (mean ± SD). Superselective intra-arterial administration of the same drug was followed by an increase of tumor intracellular pH by 0.15 ± 0.6 pH units (mean ± SD). These changes occurred prior to any changes on x-ray, computed tomography (CT), or magnetic resonance imaging (MRI). In addition to enhancing our understanding of the metabolic effects of BCNU, such changes may correlate with drug efficacy or toxicity and may be useful in guiding therapy in the future.