Duchenne muscular dystrophy is an X‐linked devastating disease due to the lack of expression of a functional dystrophin. Unfortunately, the dystrophin‐deficient mdx mouse model does not present clinical signs of dystrophy before the age of 18 months, and the role of dystrophin in fiber integrity is not fully understood. The fragility of the skeletal muscle fibers was investigated in transgenic mice expressing β‐galactosidase under the control of a muscle specific promoter. Adult mdx/β‐galactosidase (dystrophin‐negative) and normal/β‐galactosidase (dystrophin‐positive) mice were submitted to one short session of eccentric, downhill running exercise. The leakage of muscle enzymes creatine kinase and β‐galactosidase was investigated before, 1 h after, and 3 days after the running session. A significant and transient rise in the level of these enzymes was noted in the serum of mdx mice following the exercise session. Thus, the lack of dystrophin in the mdx model led to local microdamages to the exercised muscle allowing leakage of proteins from the fibers. The peak leakage was transient, suggesting that muscle fiber lesions were rapidly repaired following this short, noninvasive eccentric running session. © 1998 John Wiley & Sons, Inc. Muscle Nerve 21:567–576, 1998.