Small loops of mouse jejunum were exteriorized and heated by immersion in a bath of Krebs-Ringer salt solution. Crypts were lost in the heated regions with a half-time of approximately six hours and reached a steady level of damage by 10--16 hours. There was no recovery in crypt number for one week after hyperthermia. Using a 24 hour assay, crypt survival curves were obtained using various heating times in the temperature range 37.5 degrees C--44.5 degrees C. These curves were qualitatively similar to those resulting from radiation damage, showing a shoulder followed by exponential killing. As the temperature was increased, progressive changes in shape of the curves indicated a proportional inhibition of accumulation of sublethal heat damage combined with increased rate of expression of lethal damage. Over the temperature range 42.3 degrees C--44.5 degrees C, a linear relationship was found between the rate of crypt loss and the reciprocal of the absolute temperature. An activation energy of 600 +/- 70 kJ mole-1 was calculated using the Arrhenius equation. In this temperature range, doubling the heating time had the same effect as increasing the temperature by 1 degree C. At temperatures below about 42.3 degrees C, the tissue became relatively less sensitive to increasing the treatment time.