Abstract Recent evidence suggests that cellular sodium regulation may be abnormal in muscular dystrophy. We have measured intracellular sodium concentration (Na i) in muscles of mdx mice (a model of Duchenne muscular dystrophy) using two techniques. Na i in isolated diaphragm (measured using a microelectrode) was 13.0 ± 0.3 mM and 23.5 ± 0.7 mM (mean ± SE) in the control and mdx mice respectively. Na i in gastrocnemius muscle (calculated from extra- and intracellular volumes using serum and whole-muscle sodium concentrations) was 13 ± 3 mM and 24 ± 2 mM (mean ± SE) in control and mdx, respectively. We argue that this abnormality in mdx tissues could reflect a reduced flux through the Na K ATPase, although a contribution from increased Na leak cannot be ruled out. We also discuss possible consequences of an increased Na i: for example, raised Na i may lead to defective cell volume control in Duchenne dystrophy and the mdx mouse.