Oligodendrocytes undergo apoptosis in the white matter tracts remote from the experimental cord injury, although its significance is not understood. Our ultrastructural study, however, enabled us to speculate on its neurobiological implications. The spinal cords of male Wistar rats (4 week-old) were transected at Th11 level. At 4, 5, and 7 days after surgery the animals were transcardially perfusion-fixed. The removed cord was embedded in epoxy resin and examined by electron microscopy. Post-embedding terminal deoxynucleotidyl transferase-mediated biotinylated deoxyuridine triphosphate nick end labeling (TUNEL) method was also performed. In the degenerative dorsal column above the transection, profiles of apoptotic oligodendrocytes were always found, embedded in a space formed by dilated degenerative myelin lamellae. Often, the dilated space in the myelin sheath lacked any apparent background proteinaceous matrix. In the electron microscopic TUNEL method, these apoptotic cells were electron dense in accordance with nuclear heterochromatinization. In conclusion, in the process of Wallerian degeneration, we observed the apoptosis of oligodendrocytes in a space formed by the split myelin sheath. These degenerative cells, which were enclosed in an ultrafiltrate-filled space formed by split myelin lamellae, were reminiscent of "anoikis."