A 5 mm segment of the rat sciatic nerve was treated at 38 or 43 degrees C for 30 min using a brass thermode. This pretreatment was followed by a test heat treatment at 45 degrees C. Different intervals between the pretreatment and test treatment were studied. The effect of fractionated hyperthermia on the motor function of rat sciatic nerve was evaluated using a functional assay, the toe-spreading test. Both pretreatments led to thermal resistance of the nerve, which was maximal 24 h after the pretreatment. Thermal resistance, induced at 38 degrees C, did not show any decay over a period of 6 weeks. Thermal resistance, induced at 43 degrees C, decayed slowly, but after a 6-week interval between priming and test heat treatment thermal resistance was still observed. As the resistance induced by a mild heat pretreatment is transient, we considered this to be thermotolerance. We accounted for the thermal resistance induced by the 38 degrees C pretreatment in the calculation of the thermal tolerance ratio (TTR) after mild heat treatment at 43 degrees C. Maximal thermal tolerance was observed 24 h after mild heat with a TTR of 3.4 +/- 0.6. The TTR after a 6-week interval had declined to 1.4 +/- 0.3.