A new mouse mutant (A2G-adr) with abnormal muscle function is described and has been compared with the 129 Re dystrophic mouse. The mutation, which is due to an autosomal gene defect, results in myotonic-like spasms, progressive muscle weakness and a reduced lifespan. Affected animals were consistently lighter than normal littermates; comparison of organ weights and organ-to-bodyweight ratios indicated a slower growth rate in the mutants.