Rat cervical spinal cord was X-ray irradiated at doses of 15, 18, 20 and 26 Gy. Ninety days later, approximately the same part of the spinal cord was heated at 42.3 +/- 0.4 degrees C for 50, 60, 75 or 90 min by means of a 434 MHz microwave applicator. After treatment, animals were observed over a period of 18 months for expression of neurological complications. These complications could either be the result of the heat or of the radiation treatment. The time course showed three distinct peaks in the incidence of neurological symptoms. The first peak was due to the acute response to hyperthermia. The ED50 value for neurological complications one day after treatment at 42.3 +/- 0.4 degrees C was 74 +/- 2 min. Previous X-ray irradiation of the spinal cord with 18, 20 and 26 Gy reduced the ED50 to 57 +/- 7, 65 +/- 4 and 55 +/- 5 min (12-26% of control), respectively. Recovery from heat-induced neurological complications was diminished in previously irradiated animals. The second peak (150-300 days after X-rays) concerned the expression of "early delayed" radiation damage. Hyperthermia given 90 days after irradiation did not influence either the percentage of animals with paralysis or the latent period. Neurological symptoms developing after day 300 were due to the "late delayed" radiation response. No significant difference was observed in the data on paralysis induced by radiation alone or radiation followed by heat. The late radiation-induced minor neurological symptoms were, however, influenced by retreatment with heat.