The aim of the present study was to establish whether alterations in sarcoplasmic reticulum function are involved in the abnormal Ca²⁺ homeostasis of skeletal muscle in mice with muscular dystrophy (mdx). The properties of the sarcoplasmic reticulum and contractile proteins of fast- and slow-twitch muscles were therefore investigated in chemically skinned fibres isolated from the extensor digitorum longus (EDL) and soleus muscles of normal (C57BL/10) and mdx mice at 4 and 11 weeks of development. Sarcoplasmic reticulum Ca²⁺ uptake, estimated by the Ca²⁺ release following exposure to caffeine, was significantly slower in mdx mice, while the maximal Ca²⁺ quantity did not differ in either type of skeletal muscle at either stage of development. In 4-week-old mice spontaneous sarcoplasmic reticulum Ca²⁺ leakage was observed in EDL and soleus fibres and this was more pronounced in mdx mice. In addition, the maximal Ca²⁺-activated tension was smaller in mdx than in normal fibres, while the Ca²⁺ sensitivity of the contractile apparatus was not significantly different. These results indicate that mdx hindlimb muscles are affected differently by the disease process and suggest that a reduced ability of the Ca²⁺-ATPase to load Ca²⁺ and a leaky sarcoplasmic reticulum membrane may be involved in the altered intracellular Ca²⁺ homeostasis.