We report on the cochlea of a novel metallothionein-I (MT)/RFP-RET transgenic mouse model with severe systemic melanosis. Electron microscopy revealed that these transgenic mice possess abundant quantities of melanin in the intermediate cells of the stria vascularis. High performance liquid chromatography analysis indicated that cochleae of these transgenic mice contained about twice as much eumelanin as cochleae of control C57BL/6 mice and that the amount of pheomelanin was approximately equal in these two strains. Auditory brainstem responses at 2, 4, 8, and 16 kHz were not significantly different between transgenic and control mice. This is the first report on a mouse model of overproduction of cochlear eumelanin, and our results suggest that this transgenic mouse is an excellent model for investigating the effects of overexpression of cochlear eumelanin. In addition, we provide evidence that eumelanin overproduction in the cochlea does not affect normal hearing.