Objective To investigate the expression of GFAP in rat hypothalamus after acute heat stress. Methods The rats were caged in a experimental incubator for 60 minutes, the temperature within the incubator was adjusted to 24℃,34℃,38.5℃ or 42℃, the humidity was 60%. Single anti-GFAP immunohistochemical (ABC) method and antiFos and GFAP double immunohistochemical method were used to observe the expression of GFAP in hypothalamus in different ambient temperatures after heat stress. Results The GFAPpositive cells were rare in hypothalamus at 24℃, however it was increased in many nuclei(anterior hypothalamic area, paraventricular nucleus,arcuate nucleus, suprachiasmatic nucleus and supraoptic nucleus)at 34℃ and peaked when ambient temperature was 38.5℃, and then decreased. However