Muscles of the mdx mouse lack dystrophin, a cytoskeletal protein. Mdx fibres exhibit an increased fragility to hypo-osmotic shock and to forced lengthening, an abnormal opening time of stretch-sensitive calcium channels. The question of a chronic elevated [Ca2+]i value is a matter of controversy. We have analysed Ca homeostasis in smooth and skeletal muscles from the adult mdx mouse. The wall of the vas deferens was loaded with the fluorescent Ca indicator Fura-2-AM (cell-diffusible). Resting [Ca2+]i was measured after changes of the electrochemical potential for Ca2+ and after KCl or electrical stimulations. In no instance was a difference observed between these and similar muscles from control mice. Single striated fibres were isolated by collagenase treatment of the flexor digitorum brevis muscle and loaded with Fura-2-AM. The value of resting [Ca2+]i was measured using an in situ calibration procedure which took account of Ca buffering by Fura-2. A chronic increase of cytosolic Ca2+ was not confirmed. The expression of the intracellular Ca-binding protein, parvalbumin, was measured. It increased by about threefold in fast mdx muscles (tibialis anterior) but remained undetectable in the soleus. It is hypothesized that parvalbumin helps to maintain [Ca2+]i within normal values. This hypothesis will be discussed in connection with dystrophy phenotypes in mutant dogs and in human patients.