Objective
To evaluate the changes of motor unit number estimation (MUNE) and the amplitude of compound muscle action potential (CMAP) according to severity of nerve injury in rat.

Method
The sciatic nerve was compressed with two different severity (mild, severe) in 20 rats each. MUNE was performed in gastrocnemius of rat using the Poisson statistics. MUNE and amplitude of CMAP were compared according to severity of injury at two and four weeks after injury.

Results
The MUNE and amplitude of CMAP were significantly diminished at two and four weeks after nerve injury in severely injured rat. Both values were also diminished at two weeks after injury in mildly injured rat. The amplitude of CMAP was slightly increased at four weeks after mild injury despite of further decrement of MUNE.

Conclusion
The MUNE and amplitude of CMAP changed differently according to severity of nerve injury in rat, which might represent different denervation and reinnervation process.