Contraction of a vertebrate skeletal muscle fibre is triggered by electrical depolarization of sarcolemmal infoldings termed transverse-tubules (t-tubules), which in turn causes the release of calcium from an internal store, the sarcoplasmic reticulum (SR). The mechanism that links t-tubular depolarization to SR calcium release remains poorly understood. In principle, this link might be provided by the prominent slow calcium current that has been described in skeletal muscle cells of adult frogs and rats. However, blocking this current does not abolish the depolarization-induced contractile responses of frog muscle, and the function of this slow calcium current is unknown. Here we describe measurements of calcium currents in developing skeletal muscle cells of normal rats and mice, and of mice with muscular dysgenesis, a mutation that causes excitation-contraction (E-C) coupling to fail. We find that a slow calcium current is present in skeletal muscle cells of normal animals but absent from skeletal muscle cells of mutant animals. The effect of the mutation is specific to the slow calcium current of skeletal muscle; a fast calcium current is present in developing skeletal muscle cells of both normal and mutant animals, and slow calcium currents are present in cardiac and sensory neurones of mutant animals. We believe this to be the first report of a mutation affecting calcium currents in a multicellular organism. The effects of the mutation raise important questions about the relationship between the slow calcium current and skeletal muscle E-C coupling.