A 5-mm segment of the rat sciatic nerve was treated in vivo with hyperthermia (43-45 degrees C) for different times using a brass thermode. The effect of this local heat treatment on the nerve was assessed with electrophysiology and using two functional assays. Hyperthermia led to a dose-dependent decrease of motor and sensory function. Electrophysiological examination showed a decrease in amplitude of motor and reflex responses rather than a decrease in conduction velocities. Calculated ED50 values were not significantly different for the two functional and for the electrophysiological methods. Functional recovery from nerve damage took place in all cases. Measured at the same level of damage, i.e., 50% function loss, it took 14 days to recover from complete sensory function loss and 20 days from complete motor function loss. Although both motor and sensory functions were restored, 30 days after hyperthermia no responses could be detected with electrophysiology, this as a result of the thin myelin sheaths that occur upon recovery.