Model I: A portion of the monkey brain, the right occipital lobe, was exposed to 3,500 rads of orthovoltage radiation in a single dose. This demonstrated a) the delayed, 4 to 5 months, massive break in blood-brain barrier with extravasation of plasmatic fluid throughout the right hemisphere causing gross brain distortion, and b) after a protracted course, a remaining impairment in function extending beyond the irradiation lesion. Model II: The whole brain was exposed to supervoltage radiation in single doses of 1,000, 1,500, and 2,000 rads, respectively. This demonstrated a) the lack of effect from 1,000 rads; b) the wide scatter of necrotic lesions in the forebrain white matter from 1,500 rads at 6 months, followed by confluent necrosis in the cerebral hemispheres at 12 months; and c) the numerous focal lesions throughout the brain from 2,000 rads, with the lesions in the brain stem precluding survival beyond 6 months. Model III: The whole brain was exposed to fractionated doses of 4,000, 6,000 and 8,000 rads in 4, 6, and 8 weeks, respectively. This demonstrated a) the lack of effect from 4,000 rads; b) the wide scatter of small focal lesions at different stages of breakdown and repair accompanied by focal edema from 6,000 rads at 6 months, followed by more of the reparative processes including mineralization of focal lesions, and widespread telangiectasia, at 12 months; and c) with 8,000 rads, at 6 months, focal lesions similar to those from 6,000 rads, but at 12 months, gross brain loss from coalescing necrosis.