Most mechanisms of drugs which are used in brain tumor chemotherapy are well characterized: alkylation of DNA components (nitrosoureas), binding with tubulin protein resulting in metaphase arrest (vincristine), chromatid breaks and chromosome translocations (procarbazine), or inhibition of ribonucleotide reductase (hydroxyurea) [1]. These drugs exert their effects mainly during certain cell cycle phases of proliferating cells, particularly when DNA is synthesized. From this it can be assumed that the efficacy of these drugs depends on the fraction of proliferating cells. Thus it would be of great importance to estimate the proliferation rate of brain tumors which could guide chemotherapy in individual patients. Positron emission tomography (PET) measures quantitatively the in vivo tissue uptake of tracer substances. In tumors, the uptake appears to be altered in a characteristic way determined by biochemical properties of tumor tissue. Some aspects of brain tumor metabolism which are theoretically related to proliferation have been investigated with PET. In the following, the literature is reviewed with regard to: 1) tracer substances whose uptake has been thought to reflect tumor malignancy (11C-methionine, 18F-fluoro-deoxyglucose), and 2) tracers which theoretically could reflect mechanisms specifically related to DNA synthesis (11C-putrescine, ligands for peripheral benzodiazepine receptors).