The difference in the lifespan of dy and mdx mice could be due to different muscle regeneration capabilities. In mdx an involvement of bFGF in stimulating regeneration has been postulated. The aim of our work was to detect the presence, and to study the distribution, of muscular and connective tissue growth factors in mdx and dy mice at different stages of muscle pathology. From 7 to 10 weeks of age the difference between the two dystrophic mice becomes evident. At 13 weeks the dy mouse presents a predominance of fibrosis and degenerative muscular phenomena while the main pathological feature in mdx mouse is the muscle regeneration. In both animal models fibrosis proliferation is correlated to the presence of EGF and its receptor and TGF beta 1. bFGF was localized to regenerating and degenerating fibers in both dy and mdx mice. The bFGF presented a normal pattern in mdx mice at 20 weeks when regenerative and degenerative phenomena were no longer present. Our data suggest that growth factors could influence the outcome of muscular regenerative and degenerative processes.