Activity of arginase and of its isoenzymes was studied in rat brain tissue and in neurinoma tissue (strain 10-13-3) at the period of growth of the tumor in trigeminal nerve. Within the fourth day after the tumor transplantation the total activity of arginase was increased in brain and distinct alterations were found in the isoenzyme spectrum, mainly in the impaired hemisphere. The enzymatic activity was increased in the tumoral tissue within 16 days; the activation was localized in the malignant tissue and did not extent into surrounding nerves. In all the samples studied the positively charged isoenzyme I was activated, whereas the activity of the isoenzyme II was altered only slightly and usually tended to decrease. It was the activity of the isoenzyme I, which appeared to be altered in the growing tumor.