Brain protein synthesis may be evaluated in vivo by a PET three compartment methionine model. 14 human brain tumor patients were studied. Protein synthesis rate (PSR) was increased in any glial tumor even in low grades, but this increase was statistically more important in anaplastic tumor. Radiotherapy action was evaluated in two patients. Local tumoral PSR was reduced to normal brain PSR after treatment. No difference was seen in normal cortex contralateral to the lesion between pre and post radiotherapy examination. 11 C-L-Methionine incorporation measured by PET looks as a very sensitive method for studying tumor metabolism and treatment effects.