The arterial wall (abdominal aorta, splenic artery) has been investigated after irradiation with 3000 rad (3 X 1000 rad within 3 weeks) in rabbits. In addition to light microscopic techniques semi-thin sections, electron microscopic sections, permeability staining and autoradiographic techniques have been applied. These studies revealed typical lesions in the media (elastic degeneration, cellular proliferation of the smooth muscle cells) and lesions of the endothelial cells using ultrastructural techniques especially. The intima showed an increasing permeability in the irradiated area, while autoradiographically a higher mitotic activity in the media was demonstrable. The results are discussed in the light of similar atherosclerotic and X-ray induced lesions of human blood vessels.