Because stress proteins are believed to play an important role in cellular repair and survival mechanisms, we investigated accumulation of the 70-kilodalton stress protein (SP70) and its mRNA in the rat corneal epithelium after hyperthermia. In the corneal epithelium of control rats, in situ hydridization with a radioactive probe for SP70 mRNA followed by autoradiography revealed very few silver grains. Eighteen hours after the rats were subjected to hyperthermia, the density of silver grains was greatly increased and this elevated level of corneal epithelium SP70 expression continued through 50 hours after heat treatment. Immunostaining for SP70 in the corneal epithelium was consistent with the in situ hybridization, being weak and mainly confined to the basal cells in control rats and increasing by 18 hours in the heat-treated rats. At 50 hours post-heat treatment, the immunostaining was denser than control in all corneal epithelial cells, especially in the apical portions of the basal and wing cells. These results suggest that SP70 may be an important factor in the response of the corneal epithelium to adverse environmental changes.