The paper deals with morphological changes in glioblastoma multiforme following megavoltage radiotherapy (1.25 and 25 MeV). Both tumour cell death and glioblastoma multiforme tissue destruction resulted from 6,000-7,500 rads megavoltage irradiation. Depending upon the time following irradiation the histological study showed the prevalance of either acute aseptic inflammation, or coagulation necrosis, or degenerative changes leading to the formation of glial and connective tissue scars and dystrophic calcification. Glioblastoma multiforme, rich in cell elements with hyperchromatic nuclei and having a high mitotic activity, proved to be the most radiosensitive tumour. The highest radioresistance was noted in tumours with a tendency to haemorrhages and necroses.