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In affected muscles of dystrophic chickens, electromyography with needle electrodes revealed hyperexcitability to mechanical stimuli such as electrode movement (prolonged "insertion activity"), spontaneous potentials, and polyphasic motor unit potentials. In agreement with pathological findings in this and previous studies, the light breast muscles exhibited more extensive electromyographic abnormalities than the more distally placed light muscles of the wings, whereas the dark muscles of the legs showed little or no electromyographic abnormality. Electrode movement in the muscles of 5-day-old chickens evoked a discharge of high-frequency, low-amplitude potentials which continued for several seconds. In normal chickens the duration of this discharge diminished with age and reached the adult level, 1 sec, at 30 days of age. Breast muscles of dystrophic chickens first exhibited distinct electromyographic abnormalities at I6 days of age. It is possible that onset occurred earlier but was obscured by the high-frequency discharges normally observed during this period of life.