Genetically normal (C57BL/10) and dystrophic (mdx) mice underwent a 15 week endurance swimming programme (2 hours per day, 5 days per week) where animals were weighted (5% body weight) during most sessions. No significant changes were seen in the contractile properties or morphology of muscles from control mice following the exercise protocol. In contrast, the soleus muscles of exercised mdx mice displayed higher normalized tensions, and the extensor digitorum longus (EDL) exhibited longer half-relaxation times compared with sedentary mdx mice. Both the EDL and soleus muscles of mdx mice exhibited increased resistance to fatigue after endurance exercise. Sedentary mdx mice exhibited increased proportions of type I (slow oxidative) fibres in the soleus and type IIA (fast, oxidative glycolytic) fibres in the EDL compared with animals of the normal strain. In both the EDL and soleus muscles of mdx mice an even greater proportion of type I fibres was apparent following the training programme. The endurance exercise was beneficial to the mdx mice, enhancing the regeneration of function of their muscles by increasing the proportion of oxidative fibres and reducing muscle fatiguability.