There has been recent increasing interest in the involvement of superoxide radicals (O2-) and their scavenging enzymes, the superoxide dismutases, in the patho-physiology of certain diseases. Since mitochondria are significant intracellular sources of O2- and important targets of oxidant injury, determining the intracochlear localization of mitochondrial O2- scavenging enzyme may provide important insight into the pathogenesis of injury due to cochlear oxidants. In order to locate the mitochondrial O2- scavenging enzyme, manganese superoxide dismutase (MnSOD), the authors used a modified immunoglobulin peroxidase bridge sequence method to detect MnSOD in paraffin-embedded, formalin-fixed rat cochleas. Site-specific immunolocalization of MnSOD could be demonstrated in the cochlear labyrinth, suggesting that the generation of intracochlear O2- was possibly implicated in the metabolically active sites and sites rich in vascularity. This study also provided a useful probe for detecting MnSOD immunohistochemically from ethylenediamine tetra-acetic acid-treated materials without requiring an antigen retrieval procedure.