Expression of two myogenic regulatory factors, MyoD and myogenin, was studied in regenerating muscles of dystrophic mice and compared to a chemically induced regeneration process. First, the distribution of the two proteins was determined immunohistochemically at various time points after single administrations of a local anaesthetic, bupivacaine hydrochloride, which causes myonecrosis followed by regeneration. Detectable levels of MyoD appeared at 18 h and the expression reached their maximum levels at 48 h after the injection, which coincide with the stage when satellite cells are activated and start to proliferate. Myogenin became detectable in 24 h and its expression reached its highest level at 72 h after injection when newly formed myotubes appeared. The two genes were also expressed in the dystrophic muscles from dy and mdx mice which exhibit dystrophic pathological features but are associated with different phenotypes. In mdx mice the two genes were expressed at reasonably high levels in parallel with the active regenerating process, whereas in dy mice MyoD and myogenin expressions decreased as fibrosis progressed. However, MyoD was relatively more strongly expressed in the larger mature myotubes of dy mice than in those of mdx mice, suggesting prolonged regenerative activity. In dy and mdx mice, MyoD and myogenin were expressed in different quantities, indicating that these animals have distinct regenerating activities. Our findings confirm that expression of both MyoD and myogenin genes is necessary in the regenerative process for the proliferation of satellite cells (myoblasts) and for the development of early regenerating fibers (myotubes) even in dystrophic muscles.