[en] An investigation has been carried out into the potentiation of the biological effects of single doses of radiation (4MeV photons) on the spinal cord of rats by whole-body hyperthermic treatment of anaesthetized animals (42⁰c for 30 min). The animals received doses ranging from 1000 to 2900 rad, and hyperthermia was produced by placing the animals in chambers heated with warm moist air. Spinal canal temperature was monitored in other animals by surgical placement of a thermistor into the spinal canal, and these temperatures were shown to correspond to within 0.2⁰C of the rectal temperatures. Production of overt myelopathy at seven months post irradiation was demonstrated in over 50% of the animals receiving 2000 rad plus hyperthermia, whereas only one in ten rats receiving 2000 rad radiation alone showed paralysis at 10 months post irradiation. No myelopathy had developed at 9 months post irradiation in rats receiving 1000 or 1500 rad plus hyperthermia. Heat treatment plus 2600 rad caused paralysis in ten of ten animals. This decrease in the radiation tolerance of the spinal cord by hyperthermia has important implications for any clinical trials of combined hyperthermia and radiation in diseases where the spinal cord may be in the radiation field. (U.K.)