The regeneration of skeletal muscle is dependent upon proliferation and fusion of activated mononuclear muscle precursor cells. Early and specific markers of this population of activated cells are the transcription factors MyoD and myogenin. Northern analysis was used to determine levels of MyoD and myogenin mRNA in (i) muscles regenerating after experimental crush injury and (ii) in limb muscles of dystrophic mdx mice at various ages in comparison to controls. In crush-injured muscle, MyoD and myogenin mRNA increased at 24 h, peaked between 2 to 6 days, and returned to uninjured control levels by 15 days after injury. In both mdx and control mice, MyoD and myogenin mRNA levels were high in fetal muscles and decreased rapidly during the 2 weeks after birth. In mdx muscles, the mRNA levels increased significantly from about 21 days, remained high until around 40 days, and then decreased to a relatively constant yet elevated level when compared to control muscles. The elevated levels persisted to 420 days of age. The results show that this technique can be used to provide sensitive quantitative information on the size of the population of activated precursor cells in skeletal muscle. As such, it represents a novel and convenient means of measuring regenerative activity in vivo in whole muscles.