OUTLINE: Patients are assessed for viable tumor during a debulking craniotomy or stereotactic biopsy. If viable tumor is confirmed, patients undergo placement of an intracerebral microdialysis (MD) catheter. The catheter, which is perfused continuously over 24 hours with artificial cerebrospinal fluid, is placed directly into the residual brain tumor or peritumoral brain tissue for neuropharmacokinetic studies of temozolomide. At least 25 hours but no more than 72 hours after placement of the catheter, patients receive a single dose of oral temozolomide. At the same time, intracerebral MD is initiated and serial samples of dialysate (i.e., brain extracellular fluid [ECF]) are collected periodically over 24 hours for analysis of intracerebral concentrations of temozolomide by liquid chromatography/tandem mass spectrometry. Serial blood samples for measuring plasma levels of temozolomide are also drawn and plasma temozolomide levels are compared with those in the brain ECF. Additional ECF samples are obtained to assess biochemical markers of brain tumor metabolism (i.e., glucose, lactate, pyruvate, glutamate, or glycerol) at least one hour before and 24 hours after administration of temozolomide.