The mdx mouse, a model of the human disease Duchenne Muscular Dystrophy, has skeletal muscle fibres which display incompletely understood impaired contractile function. We explored the possibility that action potential evoked Ca²⁺ release is altered in mdx fibres. Action potential evoked Ca2+ dependent fluorescence transients were recorded using both a low and a high affinity Ca²⁺ indicator from enzymatically isolated fibres obtained from EDL and FDB muscles of normal and mdx mice. Fibres were immobilized either using intracellular EGTA or N-benzyl-p-toluene sulphonamide, an inhibitor of the myosin II ATPase. We found that the amplitude of the action potential evoked Ca²⁺ transients is significantly decreased in mdx mice with no measured difference in that of the surface action potential. In addition, Ca²⁺ transients recorded from mdx fibres in the absence of EGTA also displayed a marked prolongation of the slow decay phase. Model simulations of the action potential evoked transients in the presence of high EGTA concentrations suggests that the reduction in the evoked sarcoplasmic reticulum Ca²⁺ release flux is responsible for the decrease in the peak of the Ca²⁺ transient in mdx fibres. Since the myoplasmic Ca²⁺ concentration is a critical regulator of muscle contraction, these results may help to explain the weakness observed in skeletal muscle fibres from mdx mice and, possibly, Duchenne Muscular Dystrophy patients.