The development and decay of thermotolerance to the lethal effect of systemic hyperthermia was investigated in Sprague-Dawley rats. Systemic hyperthermia was induced by partial submersion of gas-anesthetized rats into a temperature-controlled water bath. Survival was determined in rats challenged for periods of 10 to 90 min at 42.5 C, 24 to 144 h after a sublethal conditioning exposure of 41.8 C for 1 h. Survival curves of various exposure times at 42.5 C lethal to 50% of the animals (LD50) were determined for the conditioned animals and compared with those obtained for simultaneously treated control (unconditioned) rats. The thermotolerance ratios (TTR-LD50 for conditioned animals at various times after sublethal conditioning exposure divided by the LD50 for the control animals) were calculated and compared with the ratios reported in the literature for cells heated in vitro and for tissues heated locally in vivo. Pretreatment of rats at 41.8 C for 1 h resulted in an increase in the LD50 when rats were challenged 24 to 96 h later, with a maximum increased noted at 48 h (56.5 vs. 25.0 min for control; TTR = 2.3). The animals remained relatively resistant to the second heat treatment at 96 h and returned to control levels of heat sensitivity by 120 h. The development and kinetics of thermotolerance to lethality induced by whole-body hyperthermia need to be considered when multiple-fraction hyperthermia treatment plans are designed.