Radiation damage to the right femoral artery was studied in mice 7--8 weeks old. Single X-ray exposures of 500 to 10 000 rad were used. The animals, except for the right upper legs, were shielded from exposure. The unexposed left femoral arteries served as 'controls'. Animals were killed at post-irradiation times of up to 120 days. Pathological changes in the irradiated arteries were scored on an arbitrary scale of 0 to 5 for individual tunica. The overall arterial damage was scored on a scale of 0 to 15. Radiation doses of 1000 to 10 000 rad produced quantitative histological changes in the three individual tunica and the degree of damage was dose-dependent. The tunica adventitia showed the maximum radiation injury, followed by tunica media and tunica intima in decreasing magnitude of injury. The combined effects on the artery followed closely the effects on media and adventitia.