We have used light microscopic histomorphometry to quantify the developmental histopathological changes induced by muscular dystrophy in the soleus and extensor digitorum longus (EDL) muscles of the mdx mouse. We find that this X-linked disease exhibits early fibre necrosis with foci of invasive cells, clustering of affected fibres, hyaline fibres, and, in the mixed soleus muscle, a progressive increase in the proportion of type 1 fibres, the mdx soleus containing 58 +/- 5% type 1 fibres by 26 weeks, compared with 27 +/- 4% in control C57BL/10 ScSn mice. This increase is not due to atrophy or slow axon reinnervation of type 2 fibres. Although only 5% of all original fibres survive by 26 weeks in the EDL, the diseased mdx fibres are continuously and successfully replaced by new fibres with internal nuclei, the affected mice thus avoiding the end-stage histopathology and physical disability characteristic of the X-linked human Duchenne and Emery-Dreifuss muscular dystrophies. Homozygous mdx mice share the life expectancy of normal C57BL/10 mice and appear behaviourly normal. The mdx mouse is therefore an excellent mammalian model in which to study the processes of muscle fibre degeneration and regeneration.