in this study we describe the early changes of the myelin sheath following surgical nerve crush. we used the freeze-fracture technique to better evaluate myelin alterations during an early stage of wallerian degeneration. rat sural nerves were experimentally crushed and animals were sacrificed by transcardiac perfusion 30 h after surgery. segments of the nerves were processed for routine transmission electron microscopy and freeze-fracture techniques. our results show that 30 h after the lesion there was asynchrony in the pattern of wallerian degeneration, with different nerve fibers exhibiting variable degrees of axon disruption. this was observed by both techniques. careful examination of several replicas revealed early changes in myelin membranes represented by vacuolization and splitting of consecutive lamellae, rearrangement of intramembranous particles and disappearance of paranodal transverse bands associated or not with retraction of paranodal myelin terminal loops from the axolemma. these alterations are compatible with a direct injury to the myelin sheath following nerve crush. the results are discussed in terms of a similar mechanism underlying both axon and myelin breakdown.