Muscle regeneration was studied by light and electron microscopy in soleus muscles of rat. After segmental crushing the number of fibres increased in some muscles within 30 days, indicating that numerical hyperplasia can take place. Locally applied Ringer solution of 60-70 degrees C caused necrosis of myofibres but left satellite cells and blood supply largely intact. Following phagocytosis, four mechanisms of regeneration were seen. (1) Lost fibres were replaced by clusters of myotubes formed by satellite cells within persisting basal lamina tubes. These clusters displaced the surrounding endomysium and looked like longitudinally 'split' fibres. (2) Viable fibre fragments fused with satellite cells. (3) Satellite cells of surviving fibres proliferated and fused to myotubes localized beneath the basal lamina. (4) Thin new fibres occurred in the interstitium. Their origin remained unknown. After 6 months the mean size of the new myofibres was normal, but the scatter of diameters was increased, central nuclei, fibre 'spliting' and branching, and fibrosis were prominent. Staining for acetylcholinesterase revealed that many fibres were short and not innervated. The similarity with dystrophic muscles in man suggested, that the most prominent histological changes in myopathic muscles may be due to attempts of regeneration.