Core-like lesions and nemaline rods were produced in type I extrafusal muscle fibers of rat soleus by Achilles tenotomy. In tenotomized solei the aggregate of motor unit activity was reduced. The continuous pattern of motor unit activity characterizing normal soleus was changed to a phasic pattern. Sciatic neurotomy or thoracic cordotomy at the time of tenotomy prevented the lesions and abolished motor unit activity in the soleus for a few days. Neuronal activation of muscle fibers appears essential for the development of core-like lesions and rods. Recoil of tenotomized muscle also seems necessary. The experimental central lesions resemble and differ from human central cores and targets. Experimental and human nemaline rods are morphologically indistinguishable. A common pathogenic mechanism in the formation of the human and the experimental lesions was not identified.