OBJECTIVE: To determine the effect of exercise in the early phase of reinnervation after sciatic nerve injuries in the rat. METHOD: Thirty six rats, Sprague-Dawley (weight, 200 to 220 g), were divided into the normal control and experimental groups. Using a haemostatic forceps, crushing injuries to the bilateral sciatic nerves were induced in the experimental group. The experimental group was further divided into exercise groups by the duration of daily swimming and initiation (duration since injury) of exercise after nerve injury (A, 2 hours/day and day 1; B, 30 minutes/day and day 1; C, 2 hours/day and week 2; D, 30 minutes/day and week 2) and non-exercise group (E). After completion of 5-week program the test results were evaluated by 1) sciatic nerve motor conduction study recorded at the gastro-soleus muscles, 2) measurement of soleus muscle tension, and 3) hematoxylin-eosin stain & alkaline ATPase stain (pH 9.4) of the soleus muscles. RESULTS: Nerve conduction study revealed significantly prolonged latency in group C and decreased amplitude in the group C, D. Peak twich tension decreased significantly in group C, D & E. Maximal tetanic tension was increased significantly in the group A compared to C. Both type I and II muscle fibers atrophied significantly in all the experimental groups compared to the normal control group with no changes of the composition of two muscle fibers. CONCLUSION: Swimming applied from the early phase after sciatic nerve injury may be beneficial in early recovery of muscle tension. Overexercise in the early stage of reinnervation, however, may hamper the functional return of the damaged muscle by nerve injury.