16 different free amino acids were determined in cerebrospinal fluid and plasma of each 5 patients with glioblastomas, meningiomas, and low grade gliomas as well as in 21 patients with lumbar disk herniations (control group). The values from the control group were in good accordance with those previously observed in normal adults of 5 studies of the literature. Significant changes were seen only in 6 of 16 amino acids. Absolute values of free CSF amino acids showed significant lower levels of valine, leucine and asparagine in the 3 subgroups whereas serine remained constantly high. The greatest changes were observed in glioblastoma and meningioma patients. Relative values gave similar results. No significant changes were found in CSF-plasma free amino acid relations. The authors conclude that changes of free CSF amino acids are due to a non-specific reaction of the brain itself to tumor growth. The different histology of the tumor does not give specific results. Determination of free CSF amino acids may help in early diagnosis of brain tumor recurrence after operation and to watch the effect of chemotherapy and radiation on brain tumor growth.