1. The effects of fasting for 48 h were investigated in CS7BL/10 (wild type) and age-matched C57BL/10 dystrophin-deficient (mdx) mice. 2. Fasting resulted in an increased percentage of necrotic fibres in muscles from the hindlimb and lumbar regions of mdx mice. The percentage of necrotic fibres of forelimb and chest muscles of mdx mice was unaltered by fasting. In wild-type mice, very few necrotic fibres were observed after fasting. 3. The necrotic changes in fasted mdx muscle were not accompanied by altered energy status as evaluated by muscle ATP and phosphocreatine concentrations. 4. A significantly decreased rectal temperature was observed in mdx but not in wild-type mice after fasting. 5. Fasting would normally be expected to cause a reduction in muscle fibre size. The high prevalence of necrosis in fasted mdx mice is therefore an unusual response that may be related to disturbance of the mechanisms which, in the fed state, compensate for the dystrophin deficiency in these animals.