OBJECTIVE To study the change of motor evoked potential of the diaphragm after graded upper cervical cord injuries and analyze the prognosis of the respiratory function after upper cervical cord injuries by MEP. METHODS The C(3, 4) spinal cord of 40 SD rats were injured with modified Allen method by weight drop force of 30 gcf, 50 gcf, 80 gcf, 100 gcf. The change of latent and amplitude of MEP of the diaphragm was observed before and after the spinal cord injuries (followed up for 1 month). At the same time, the femoral arterial blood of 0.2 ml was drawn. The pH value, the partial pressure of oxygen, the partial pressure of carbon dioxide, and the saturation of the blood oxygen were monitored. The change of the respiratory function was evaluated in the rats after cervical cord injuries. The relationship between the recovery of the respiratory function and the latent or amplitude was analyzed. RESULTS The MEP wave of the diaphragm was stable before the upper cervical spinal cord injury in the rats. It was usually composed of a positive and negative wave. The latent period and peak-peak amplitude of wave were (3.13 +/- 0.29) ms and (6.78 +/- 3.48) mv. The latent period of the diaphragms MEP in rats change with graded upper cervical cord injuries significantly, the more sever the injury, the longer the latent period. There were obvious relationship between the change of the latent period and the recovery of the respiratory function. When the latent period prolonged 101%, the respiratory function is hardly to recovery. CONCLUSIONS MEP can objectively and sensitively reflect the injury extent of the respiratory function when upper cervical cord is injured. It would be a reliable index to evaluate the long-term prognosis of respiratory function.