Induced thermal resistance was observed in the ear of the mouse after heat treatment at temperatures ranging from 41.5 to 45.5°C. For a given priming temperature both the magnitude and time course of induced resistance depended on the duration of heating. Whereas a single treatment of about 50 min at 43.5°C was required to cause necrosis in 50% of the heated ears, a priming heat treatment could induce resistance such that a second treatment of 100 min at 43.5°C was needed. This degree of induced resistance was used to investigate the relationship between the heating time and temperature to induce resistance at 24 hr after the priming treatment. The results indicate that this relationship may be different above and below 42.5°C. Heat treatments which induced the same degree of resistance at 24 hr were compared. Although similar degrees of thermal resistance were observed between 16 to 96 hr after each treatment, resistance developed more rapidly after heating at lower temperatures for longer times than at ...