The hypothesis that DNA damage is the trigger for thermotolerance in mammalian cells was tested in Chinese hamster ovary cells by looking for evidence of thermotolerance after ionizing radiation or ultraviolet light exposure. As previous studies have demonstrated that relatively non-toxic radiation exposures do not induce thermotolerance in mammalian cells (Li et al. 1976), higher doses, comparable to those used in yeast to induce thermotolerance (Mitchel and Morrison 1984), were tested in this study. Doses of this magnitude are lethal to mammalian cells, thereby precluding the use of clonogenic survival as an endpoint. We therefore used three alternative assays which are indicators of the subsequent development of thermotolerance. These were; (a) heat-induced inhibition of total protein synthesis, (b) heat-induced uptake of dansyl lysine, and (c) synthesis of heat shock proteins. Only total protein synthesis revealed evidence of a small degree of thermotolerance which occurred immediately after ionizing radiation exposure. By 4 h postirradiation the tolerance, as measured by this assay, was no longer evident. No evidence of thermotolerance was seen following UV exposure. In addition, when a large radiation dose was given either immediately before or after a heat treatment used to induce thermotolerance, there was no alteration in the level of heat-induced tolerance, despite the extensive number of DNA stand breaks caused by the radiation. Our data therefore suggest that, in mammalian cells, the type of DNA damage caused by ionizing radiation is not the trigger for the induction of thermotolerance.