Wallerian degeneration, induced after injury to a peripheral nerve, is associated with upregulation of proinflammatory cytokines, which are suggested to contribute to the development of lesion-induced neuropathic pain. In chronic constrictive injury (CCI), an animal model of injury-induced painful mononeuropathy, inhibition of synthesis, release, or function of the cytokine tumor necrosis factor-alpha (TNF) results in reduced pain-associated behavior. Here, changes of TNF content in rat sciatic nerves after CCI (days 0, 0.5, 1, 3, 7 and 14) were investigated by enzyme-linked-immunoassay. Low levels of TNF were already detectable in control nerves. Concentrations increased rapidly after CCI, with a maximum (2.7-fold) at 12 h, and remained elevated on a lower level until day 3. Baseline levels were reached again at day 14. These results indicate that TNF is produced at an early time point in the cascade of events resulting in Wallerian degeneration and hyperalgesia following peripheral nerve injury. Given that only prophylactic treatment with TNF inhibitors efficiently reduces hyperalgesia in CCI, TNF seems to contribute to the initiation of neuropathic pain in this model.