The cervical spinal cord (C5-T5) of female Wistar WU rats was irradiated with 250 kV X-rays (15-32 Gy). Heat was applied at approximately the same site 7 ± 1 min after X-rays. Early delayed paralysis of the forelegs was observed 5-10 months after treatment. The ED50 (± SE) after single-dose irradiation alone was 25.8 ± 0.4 Gy. Late delayed paralysis and paresis were observed 11-21 months after irradiation with an ED50 (X-rays alone) of 22.7 ± 0.6 Gy. The data for late paralysis, late paresis and minor neurological symptoms were pooled resulting in an ED50 (± SE) of 20.6 ± 0.7 Gy. Hyperthermia enhanced the radiation response. Thermal enhancement ratios (TER) in the early delayed response after a 30 min treatment with 41.1 ± 0.4°C, 42.1 ± 0.4°C and 42.9 ± 0.4°C were 1.07 ± 0.08, 1.17 ± 0.08 and 1.12 ± 0.04 respectively. For the late delayed radiation response concerning paralysis and paresis the TER after 30 min at 41.1°C and 42.1°C were 1.25 ± 0.10 and 1.31 ± 0.07, respectively. The latent period for paralysis was not significantly affected. Pathological examination of the spinal cord after combined treatment of X-rays and hyperthermia showed focal demyelination with white matter necrosis and vascular injury in animals as an indication of early delayed and late delayed paralysis, respectively. This was not different from histopathological changes observed after irradiation alone.