This study was conducted to clarify the effects of vibration on the peripheral nerves. Rat tails were exposed to vibration (acceleration 56.9 m/s2, frequency 60 Hz, amplitude 0.4 mm for two or four hours daily, six days a week. The maximum motor conduction velocity (MCV), the amplitude of evoked response, and the motor distal latency were measured on rat tail nerves every two months. Thin sections of tail nerves were examined under the electron microscope after 200, 500, and 800 hours of vibration. Neurophysiological and ultrastructural changes in tail nerves increased with the dose of vibration. In the groups exposed to vibration the MCVs were significantly reduced after a vibration time up to 400 hours, whereas the motor distal latency was not delayed significantly until 600 vibration hours. The ultrastructural changes were (1) detachment of the myelin sheath from the axolemma, (2) constriction of the axon, (3) protrusion of the myelin sheath into the axon, (4) accumulation of vacuoles in paranodal regions, and (5) dilatation of the Schmidt-Lanterman incisures. The ultrastructural changes induced by vibration in the paranodal regions and myelin sheaths were possibly responsible for the reduction in MCVs.