OBJECTIVE: To investigate the histopathological findings of soleus muscle in relation to the severity of injury and the duration of treadmill exercise in rats with sciatic nerve damage. METHOD: Sciatic nerve of seventy rats was compressed with haemostatic forceps. The experimental group was divided into 4 subgroups according to the intensity and duration of injury: group 1, first degree compression for 5 seconds; group 2, first degree for 30 seconds; group 3, third degree for 5 seconds; and group 4, third degree for 30 seconds. Treadmill exercise was done for either 30 minutes or 2 hours a day, 5 days a week for 4 weeks. Histochemical study of soleus was done before nerve compression and 1 week, 4 weeks after compression. RESULTS: The fiber diameter of soleus was larger in the experimental group at 4 weeks (p<0.05). The intensity of injury had greater impact on the recovery of fiber diameter than the duration. Thirty minute exercise was seen to have a earlier recovery of fiber diameter than 2 hours. CONCLUSION: These results may provide the basic data to clarify the neurological recovery in relation to the severity of injury, and to help establishing adequate duration of exercise after nerve damage.