The medial popliteal nerve was sectioned in adult rats, and reinnervation was permitted after nerve reunion. Reinnervation was observed in soleus (slow twitch) and plantaris (fast twitch) muscles for periods of 4-21 months after the initial operation. 2. In the reinnervated muscles, isometric twitch time to peak (contraction time) and time to half-relaxation were longer in soleus than plantaris in every muscle examined. 3. Some of the muscles were stained for actomyosin ATPase activity. The proportion of type I fibres (presumed slow) was at least twice as high in soleus compared with plantaris muscles. The proportion in the latter was similar to that predicted from random reinnervation. 4. The effects of differing proportions of fast and slow fibres on the twitch contraction and half-relaxation times were modelled, and the experimental twitch data were found to be compatible with the proportions of fibre types in individual muscles. 5. It is speculated that the mechanism accounting for the higher proportion of slow fibres in soleus muscle probably does not involve selective reinnervation. It is suggested that fast motoneurones may be transformed to slow by a trophic chemical released by slow muscle fibres; such a process would be more extensive in predominantly slow twitch soleus than a fast muscle, and the resulting decrease in the proportion of fast motoneurones would account for the less extensive conversion in soleus.
