The phenomenon of thermotolerance in mammalian cells has been extensively documented in the literature. Because of its potential clinical importance as well as the fundamental biological interest, we pursued additional studies investigating pH and nutritional effects. Split-dose experiments were performed using plateau phase Chinese hamster HA-1 cells. The effects of the nutritional environment during the initial 43 degrees incubation and the second 43 degree treatment on the induction of thermotolerance were studied by comparing survival in full medium at pH 7.4 or in Hanks' balanced salt solution at pH 6.7. In additional experiments, we examined the effect on thermotolerance of changing the temperature of the initial treatment to 41 degrees. Thermotolerance was induced independently of the nutrient conditions of the first treatment. However, survival at the time of maximum expression of thermotolerance depended primarily on the duration and temperature of the initial treatment. Temperatures of 43 degrees or higher inhibited the development of thermotolerance during the first heat exposure. In contrast, if the initial exposure was at 41 degrees, thermotolerance was almost fully expressed by the end of this initial treatment. Changing pH in Hanks' balanced salt solution from 6.7 to 7.4 did not affect survival. On the basis of these and other data, we suggest that thermotolerance can be divided into three complementary and sometimes competing processes: an initial event ("trigger"); the expression of resistance ("development"); and its disappearance ("decay"). Trigger is induced at all hyperthermic temperatures while development requires a permissive temperature, i.e., less than 43 degrees for HA-1 cells. This model can provide plausible interpretations of several currently puzzling aspects of the survival kinetics of heat-exposed mammalian cells.