The induction of the heat shock protein, HSP72, was studied in the cochlea of guinea pigs and rats subjected to a hyperthermic stress. Analyses were done by immunoblotting and immunocytochemistry at 6 and 12 h after heat shock, using a commercially available monoclonal antibody (Amersham), and by in situ hybridization 1 h after heat shock using an oligonucleotide probe. In guinea pig immunoblots of the cochlea, HSP72 was present in both unstressed and heat stressed animals and immunocytochemistry did not reveal any difference of staining between them. As opposed to guinea pig, HSP72 was not found in unstressed rat cochlea. Heat shock induced HSP72 expression in most inner ear tissues of the rat examined by immunoblotting. Immunocytochemistry and in situ hybridization localized HSP72 synthesis in ganglion neurons, Schwann cells, spiral limbus, spiral ligament and stria vascularis. The strongest immunoreactivity and highest density of silver grains were seen in the stria vascularis. All blood vessels were strongly immunoreactive and were outlined with silver grains. These results show that HSP72 synthesis can be induced by hyperthermia in rat cochlea and suggest that this protein could be a useful marker for assessment of the effects of specific stresses in this organ.