Glutathione-S-transferase (GST) has been found to conjugate glutathione (GSH) to diverse electrophiles and play a major role in the detoxification of alkylating and platinating agents. However, there is little information regarding the pattern of GST expression in the cochlea. Although cisplatin is ototoxic, its effect on GST in the cochlea is unknown. In the present study we investigated the pattern of GST expression in control and cisplatin-treated cochleas by using the laboratory rat as animal model. Sixteen mature rats were randomly assigned to control or cisplatin groups. After treatment, cochleas were procured and tissues stained by immunohistochemical methods to detect the presence of GST. Optical density measurements were determined to gauge intensity of GST immunostaining. Strong positive GST immunostaining was demonstrated in all cochleas, with the most intense staining found in the spiral ligament and the least in Reissner's membrane. Mean optic density scores were lower for cisplatin-treated cochleas than for control cochleas in all areas analyzed. These findings showed that GST is expressed throughout the rat cochlea, with cisplatin treatment causing its decreased expression.