Abnormal impulses in peripheral nerves play a critical role in neuropathic pain syndromes. The voltage-gated Na+ channels that underlie the action potential are main targets for clinically useful drugs in the pain therapy. Systemic tramadol has been shown to have clinical efficacy against some forms of neuropathic pain. Therefore, we investigated the mechanisms of action of tramadol by an in vitro model by sucrose-gap technique. Tramadol produced concentration-dependent and frequency-dependent decrements in CAP amplitude. Also, injured nerves were more sensitive to tramadol. Tramadol decreased the amplitude of the delayed depolarization and the hyperpolarizing afterpotentials. In conclusion, blocking potencies of small concentration tramadol on the delayed depolarization and hyperpolarizing afterpotential in regeneration period may be contributed for understanding of the action mechanisms of tramadol on neuropathic pain therapy.