In our search for genes up-or down-regulated genes in the mdx mouse model for Duchenne muscular dystrophy, we isolated a down-regulated mitochondrial DNA clone. In addition to this clone, all protein-coding mitochondrial genes tested had tissue-specific and age independent down-regulated expression. This implied mechanisms at the RNA level since no change in the mitochondrial DNA contents were detected. Cytocbrome c oxidase activity showed the same range of down-regulated expression. These data provide a molecular basis for energetic metabolism modifications in mdx mice.