The transcription of heat shock genes in response to physiological stress requires activation of heat shock transcription factor (HSF). Although the transcriptional response is most commonly induced by temperature elevation, the biochemical events involved in HSF activation in vivo can also be triggered at normal physiological temperatures by chemicals that inhibit metabolic processes. We have used a HeLa cell-free system in which HSF DNA-binding is activated by conditions that affect protein conformation, including increasing concentrations of hydrogen ions, urea, or nonionic detergents. Treatment with calcium ions also results in a concentration- and time-dependent activation of HSF in vitro. Pretreatment with each of these biochemical conditions reduces the temperature dependence for HSF activation in vitro. These results suggest that HSF is activated either directly by undergoing a conformational change or indirectly through interactions with unfolded proteins.