Recently, many studies have demonstrated the induction of stress proteins in the mammalian nervous system under various pathological conditions. These altered genetic programs may function to protect individual cells against stressful conditions. However, little is known about the molecular mechanisms regulating these stress responses in animals. We report here the activation of a heat shock factor (HSF) in the rat brain during cerebral ischemia or after heat shock. Gel mobility shift assays revealed an increase in DNA binding activity to the heat shock element (HSE) during the early phases of ischemia. Supershift experiments using specific antisera against HSF1 and HSF2 showed that the ischemia-induced HSE-binding activity was mainly due to HSF1. In the heat-shocked brain, HSF1 was also activated, and the HSE-binding activity was higher in the cerebellum than in the cerebral cortex or hippocampus; Western blot analysis also showed that HSF1 was more abundant in the cerebellum than in the other two brain regions. Our results indicate that heat shock gene transcription is regulated by the activation of HSF1 in both cerebral ischemia and heat shock, and that different brain regions display differential sensitivities in their stress response. The cellular signals for heat shock gene transcription under in vivo pathological conditions will also be discussed.