Although HSP 72 is the most conserved and best studied among HSP family, another high molecular weight heat shock protein, HSP 90 may also have an important role in cellular protection against stresses. Mammalian HSP 90 is a very adundant protein in cells grown under normal conditions and its synthesis increases after heat shock treatment(Welch et al, 1983). The purpose of this study was to investigate the expression of HSP 90 in normal cochlea and whether noise exposure could increase the synthesis of HSP 90. Immunocytochemical analyses were used to detect HSP 90 expression in the cochlea using anti-HSP 90 monoclonal antibody (StressGen Co.). Immunoreactive staining was observed in surface preparations of the cochlear spiral. A constitutive (non-induced) level of HSP 90 immunoreactivity was seen in the non-noise exposed normal animals, in both inner and outer hair cells. An increased intensity in HSP 90 immunoreactive staining of inner and outer hair cells was seen five and six hours after noise overstimulation (110dB SPL). HSP 90 immunoreactive staining, both constitutive and induced, is been in both inner and outer hair cells. The increased expression of HSP 90 after noise overstimulation may have a role in the protection of hair cells from acoustic overstimulation. HSP 90 is believed to complex with many steroid hormone receptors including the stress related glucocorticoid receptors. It will be interesting to determine the relationship between HSP 90 and steroid receptors in the cochlea and the functional roles of HSP 90 in the cochlea.