The muscle action potential evoked by the facial nerve stimulation was investigated in 31 normal subjects and in 122 patients with facial palsy. The normal finding of this nerve conduction study was described in detail, and the results of the qualitative and quantitative analysis of the nerve conduction patterns obtained from the patients were presented. The following conclusions were derived from the present study. 1. The duration, size, threshold, latency, and relationship between the response and the stimulus intensity were the important factors involved in the diagnosis of the denervation in the facial nerve. 2. On the basis of the analysis of these factors, the grade of denervation could be differentiated into no denervation, slight denervation, moderate denervation, and severe denervation. The prognosis of the palsy largely depended upon the grade of denervation. 3. The nerve conduction study could reveal minor denervation which might be overlooked by the conventional nerve excitability measurement and 4. In the cases with moderate or severe denervation, decompression surgery facilitated earlier and better recovery of the facial movements. 5. The nerve conduction study was proved to be a valuable adjunct to the conventional nerve excitability measurement, S-D curve, and electromyography. The prognosis of facial palsy as well as the indication of surgical treatment could be established by careful evaluation of the results of different electrodiagnostic examinations including nerve conduction study.