1. Isolated extensor digitorum longus muscles from control C57BL/10 and mutant dystrophin-deficient C57BL/10 mdx mice have been studied in vitro to determine whether dystrophin deficiency influences the susceptibility of muscle to contractile activity-induced damage. 2. mdx muscles were found to release reduced amounts of intracellular creatine kinase compared with control tissue in response to excessive contractile activity with or without simultaneous stretching of the muscle to 130% of its resting length. 3. In contrast, prostaglandin E2 release from mdx muscle was elevated compared with control tissue in response to either form of contractile activity or to treatment with the calcium ionophore A23187. 4. These results do not support the hypothesis that dystrophin-deficient muscle is more susceptible to damage induced by contractile activity, but suggest that dystrophin deficiency influences the activity of muscle membrane phospholipase enzymes.