Lumbar or cervical regions of the guinea-pig spinal cord were irradiated with a single dose of 250kV X-rays. The latency for paralysis, whether of hind- or forelimbs, and the histopathology of the radiation-induced cord lesions depended critically on the radiation dose. There were definite but only minor differences between the reactions of lumbar and cervical cord to the same radiation dose. After 30 or 40 Gy there was white matter necrosis but after 20 Gy widespread demyelination associated with vacuolar spaces occurred. After irradiation of the lumbar cord with 30-40 Gy, the lesions in the guinea-pig differed from those reported in the rat. White-matter necrosis in the guinea-pig cord was only occasionally associated with spinal nerve root necrosis, whereas in the rat, nerve-root necrosis with sparing of the white matter was the main lesion. After 20 Gy to the cervical or lumbar cord the guinea-pig showed widespread demyelination and vacuolation whereas in rats vascular lesions were the main result.