The brains of x-irradiated and repeatedly UV-irradiated rats and mice show a considerable increase in number of periventricular glial cells which contain copious granular material rich in thiol groups. Electron-microscopically this material is composed of dense bodies showing peroxidase activity. The appearence of the thiol-rich peroxisome-like organelles in the periventricular glial cells during the postirradiation period is interpreted as a reaction to circulating radiotoxins of organic peroxide type.