The time-course of thermal damage to the spinal cord was studied after hyperthermia of the cervical region in the rat. Local 434 MHz microwave heating of the spinal cord (cervical 5-thoracic 2) was obtained by using a ring-shaped applicator. Heat treatment at 42.9 degrees C (+/- 0.4 degrees C) for 38 min resulted in neurological symptoms, ranging from uncoordinated use of the forelegs to paralysis and death in 90 per cent (28/31) of the animals. Histological sectioning of the treated part of the spinal cord was performed immediately, 4, 24 h and 3, 7, 14 and 28 days after hyperthermia. Immediately and 4 h after treatment, neurons in the grey matter of the cord were affected and vacuolization was observed in the white matter. In animals with paralysis 1 day after treatment, we noticed neuronal degeneration, myelin pallor and sometimes haemorrhagic foci in white and grey matter. As a reaction to the thermal injury, gliosis was observed and invasion of macrophages and lymphocytes (day 3-14). Animals which had severe neurological symptoms at day 1 after hyperthermia, but had recovered completely 28 days after treatment, showed focal scar formation and demyelination in the spinal cord. The observed neurological complications correlated well with the localization of the observed histopathological changes in the cervical spinal cord.