Extracerebral tumors of posterior fossa often lead to dysfunction of the brain stem, however the degree of brain stem’s damage is determined most often on the basis clinical data only. Magnetic Resonance Spectroscopy (MRS) allows to study biochemical processes and determine chemical content of the cerebral tissues noninvasively. Changes in brain stem’s perifocal zone in patients with acoustic neuromas and subtentorial meningiomas have been studied. Basic metabolite’s level’s analysis of N-acetyl-asparatate (NAA), creatine, choline and lactate have been conducted. Changes in brain stem’s metabolism depend on duration of the disease and the conditions of the patient on admission have been revealed. However the change in metabolite’s level and their role in the brain in case of pathology and in normal conditions need further investigations.