BACKGROUND: Neuropathic pain is part of the symptom complex known as peripheral neuropathy. Sensory loss, muscle weakness, atrophy, and decreased tendon reflexes are more common than pain in neuropathic disease. Recently, Bennett and Xie reported that when the sciatic nerve of a rat is loosely ligated, the rat develops pain syndrome similar to that observed in neuropathic pain states in a human. Anatomical and physiological studies to date indicate that the major pathological finding in large diameter myelinated fibers distal to the ligatures was a complex loss of response while in small myelinated fibers there were was only subtle changes. However, a more extensive analysis of the various nerve fiber groups in the damaged sciatic nerve is required for a better understanding of the pathophysiology of the present neuropathy. METHODS: To evaluate the damage and regeneration of all caliber of peripheral nerve, we performed an electron microscopic analysis of the sciatic nerve after four loose ligatures were applied. Cross- sectional photomicrographies of regions distal to the ligatures were studied. A peripheral mononeuropathy was produced in adult rats by tying 4 ligatures loosely around the common sciatic nerve. The distal part of the ligated common sciatic nerve was severed in 2 rats of each group at 1 day, 3 days, 1 week, 2 weeks and 4 weeks respectively. The severed nerves were prepared for electron microscopic examination and pathologic changes were observed under the electron microscope. RESULTS: The ultrastructural changes after ligature application were as follows: At 1 day, the axon of A-beta fiber was shrunken and detached from the myelin sheath. C-fibers were mildly edematous and A-delta fibers appeared to be normal. On the 3rd day, the axoplasm of A-beta fibers was more shrunken, containing swelling of microorganelles and irregularly thickened myelin sheath. C-fiber showed some degrees of degeneration. A-delta fibers revealed mild degeneration and interstitial edema was also noted. At 1 week, the myelin sheaths of A-beta fibers were severely irregular in appearance with marked axonal loss. Many myelin fragments were phagocytosed in the cytoplasm of adjacent Schwann cells. At 2 weeks, A-beta fibers predominantly disappeared and many fragmented myelin sheaths were ingested in the Schwann cell. In some areas, A-beta fibers partially regenerated, which involved remyelination and an increase in the numbers of microorganelles of the Schwann cells. C-fibers were also regenerated. At 4 weeks after sciatic nerve ligation, A-beta fibers regenerated and myelin ovoids were noted within the axoplasm of the A-beta fibers. Myelin ovoids were found in the Schwann cell cytoplasm. A-delta fibers and C-fibers appeared ultrastructurally well-regenerated and had a relatively normal distribution. CONCLUSIONS: We found that maximal nerve degeneration was observed at 2 weeks after sciatic nerve ligation, thereafter, nerve regeneration was noted at 4 weeks after sciatic nerve ligation.