OBJECTIVE: To evaluate the neurophysiological changes after intramuscular botulinum toxin A (BTX-A) injection in normal adults. METHOD: Nine subjects were studied by electrophysiological measurements before and after the injections. BTX-A (5 IU, Botox , Allergen, USA) was injected in the extensor digitorum brevis (EDB) muscles. Thereafter, electrophysiological measurement was followed up during 6 months. RESULTS: The compound muscle action potential (CMAP) amplitude of injected EDB muscle decreased significantly for 8 weeks, a maximal decrement at 4 weeks after injection. CMAP peak area changes over time were nearly identical to those of CMAP peak amplitudes. The first/fourth amplitude change of CMAP with 3-Hz repetitive nerve stimulation decreased significantly for 8 weeks and the amplitude following post-exercise activation increased significantly after injection. There were no significant changes in F-wave amplitude and latency. CONCLUSION: Serial electrophysiological measurements after intramuscular BTX-A injection into EDB provide useful information for the neurophysiological change after injection.