ELECTROMYOGRAPHY, long used in physiologic research, has recently emerged as a clinical diagnostic method with a high degree of accuracy and widespread clinical application. With the recent advances in electronics it is now possible to produce commercially a sensitive instrument which utilizes a needle electrode placed directly into a denervated muscle and which records characteristic action potentials by means of a cathode ray oscilloscope. Such an instrument1has been used in this study to determine the exact location of lesions causing compression of spinal nerve roots. This localization depends on the accurate detection of the action potentials of denervation fibrillation, whereas heretofore electromyographic localization of a lesion of a single nerve root has depended on the detection of fasciculation by the use of percutaneous electrodes, a much less accurate method (page 96). Visible fibrillation of a denervated muscle was first described by Schiff in 1851, when he sectioned the