Using in vivo 31P NMR spectroscopy, the early postnatal development of lower hindleg muscles of normal and dystrophic mice was investigated. Ratios of phosphocreatine and inorganic phosphate, and of phosphocreatine and ATP increased exponentially during normal postnatal growth and differentiation. In dystrophic skeletal muscles, however, the ratios were already considerably lower during the early postnatal period, before histopathological features were observed. The ratios remained lower, relative to normal muscles, at least into the young adult stage. A deficiency in the sequestering of creatine or a defect in the phosphocreatine shuttle is proposed to explain the pathological features observed in this disorder.