The soleus muscle of young adult (5 months), adult (10-11 months) and old (23 months) male Wistar rats was unilaterally denervated for a period of 3 weeks and studied with regard to enzyme-histochemical and morphometrical properties. Denervation caused a marked atrophy of all fibres, irrespective of age and enzyme-histochemical type. Fibres having myofibrillar ATPase staining characteristics intermediate to type I and type IIA fibres increased in number in all age groups and a reduction in the number/proportion of type I fibres was found in adult and old animals. These results indicate that a slow-to-fast shift in myofibrillar properties as a consequence of denervation, shown to occur in the soleus muscle of young animals, also takes place in old age. This supports the view that atrophic fibres with intermediate myofibrillar ATPase staining characteristics, and possibly also atrophic IIA fibres, seen in old soleus muscle could be type I fibres which have undergone a transformation in response to the age-related denervation process.