The characteristics of the surface recorded F responses including minimal latency, mean latency, amplitude, duration, area and shape were investigated in the posterior tibial nerves of rabbits. During a train of 100 stimuli, each F wave was divided by the recurrence of shape into repeaters and nonrepeaters. There was no significant difference between the parameters of repeaters and nonrepeaters such as mean latency, minimal latency, amplitude and area. These findings suggest that the associated discharges of motor neurons generating the repeaters of F wave represented no definite distinctions from that of nonrepeaters in the aspect of the neuron size or the number of neurons contributing to each parameters of F wave. After the sciatic nerve was exposed, it was minimally injured by compression and then F responses were studied. Mean latency and minimal latency were prolonged, and amplitude and area were diminished in post-injured group. The frequency of repeater waves was increased but was not a useful parameter in the assessment of injury model due to a wide range of normal value.