The myoplasmic free [Ca²⁺] transient elicited by an action potential (Δ[Ca²⁺]) was compared in fast‐twitch fibres of mdx (dystrophin null) and normal mice. Methods were used that maximized the likelihood that any detected differences apply in vivo. Small bundles of fibres were manually dissected from extensor digitorum longus muscles of 7‐ to 14‐week‐old mice. One fibre within a bundle was microinjected with furaptra, a low‐affinity rapidly responding fluorescent calcium indicator. A fibre was accepted for study if it gave a stable, all‐or‐nothing fluorescence response to an external shock. In 18 normal fibres, the peak amplitude and the full‐duration at half‐maximum (FDHM) of Δ[Ca²⁺] were 18.4 ± 0.5 μm and 4.9 ± 0.2 ms, respectively (mean ±s.e.m.; 16°C). In 13 mdx fibres, the corresponding values were 14.5 ± 0.6 μm and 4.7 ± 0.2 ms. The difference in amplitude is statistically highly significant (P= 0.0001; two‐tailed t test), whereas the difference in FDHM is not (P= 0.3). A multi‐compartment computer model was used to estimate the amplitude and time course of the sarcoplasmic reticulum (SR) calcium release flux underlying Δ[Ca²⁺]. Estimates were made based on several differing assumptions: (i) that the resting myoplasmic free Ca²⁺ concentration ([Ca²⁺]_R) and the total concentration of parvalbumin ([Parv_T]) are the same in mdx and normal fibres, (ii) that [Ca²⁺]_R is larger in mdx fibres, (iii) that [Parv_T] is smaller in mdx fibres, and (iv) that [Ca²⁺]_R is larger and [Parv_T] is smaller in mdx fibres. According to the simulations, the 21% smaller amplitude of Δ[Ca²⁺] in mdx fibres in combination with the unchanged FDHM of Δ[Ca²⁺] is consistent with mdx fibres having a ∼25% smaller flux amplitude, a 6–23% larger FDHM of the flux, and a 9–20% smaller total amount of released Ca²⁺ than normal fibres. The changes in flux are probably due to a change in the gating of the SR Ca²⁺‐release channels and/or in their single channel flux. The link between these changes and the absence of dystrophin remains to be elucidated.