The myofibrillar ATPase reaction was utilized to determine the relative proportion of type II fibres in reinnervated soleus muscle 6 months after transection and reunion of the nerve at various distances from the muscle. In self-reinnervated soleus muscle, a highly significant decrease in the percentage of type II fibres from 10.5 ± 1.6% in normal muscle to 0.7 ± 0.4% was noted. In randomly reinnervated soleus muscle the percentage of type II fibres gradually increased with the distance of the site of nerve transection from the muscle. After transection and reunion of the muscular branch of the tibial nerve, the type II fibres constituted 34.0 ± 1.5% and that of the reinnervated soleus muscle after transection and reunion of the sciatic nerve stump was 73.7 ± 1.7% of the total fibre population. Different factors which might be responsible for the observed differences in the degree of cytochemical transformation of muscle fibre types in the process of reinnervation are discus