Introduction2 The use of roentgen rays in the treatment of brain tumors has at one time or another aroused great interest both in the neurosurgeon and radiologist. At various periods more or less hopeful claims have been made for the roentgen therapy of brain tumors. Some have felt that in certain types of tumors this mode of treatment offered a real hope as against surgery. On the whole, however, the general trend has been to use roentgen therapy as an adjunct to surgical treatment, particularly in the case of gliomata which have been found on exposure to be inoperable and inaccessible. Conflicting reports have resulted from this tendency, and we are in more or less of a dilemma concerning the efficacy of the roentgen ray in prolonging the life of individuals with inoperable gliomata. With recent advances in our knowledge of the gliomata and of their varied structure, further opportunity is offered to study the several types of tumors of this sort, in order to determine their sensitivity to roentgen rays. This is all the more important because most of our studies have been confined to a consideration of the clinical improvement following surgery and roentgen therapy without due appraisal of the actual changes to be noted in the irradiated tissue itself, changes which might be attributed to irradiation. We have attempted, therefore, in this study to determine what effects the roentgen rays have upon gliomata of different types, and at the same time to evaluate the changes wrought in the normal brain tissue in irradiated cases.