Introduction FASCICULATION of muscle occurs most often in diseases of the spinal cord and roots, particularly in such motor neuron degenerations as amyotrophic lateral sclerosis (ALS). It seems established, however, that fasciculation in a muscle is not abolished by spinal1-3 or peripheral nerve anesthesia.3-6 Except for one unusual case,7 fasciculation is abolished by curarization, which suggests that the locus of hyperexcitability is at a neuromuscular junction and spreads antidromically to activate some or all of the motor unit.3 Denervation by means of nerve resection in three ALS patients abolished fasciculation 8, 11, and 12 days after the operation.8 This result correlates with the time-course of degeneration in the distal nerve branches and provides support for a peripheral axon-reflex mechanism of fasciculation. On the other hand, an electromyographic (EMG) characteristic of fasciculation in ALS and related diseases is bizarre, giant action potentials9,10 which may be