The purpose of this study was to investigate the actions of lamotrigine and levetiracetam on the conduction properties of isolated rat sciatic nerves in-vitro. Compound action potentials from rat sciatic nerves were recorded using a sucrose-gap technique with single and repetitive stimulation. Lamotrigine, at 0.01 to 1 mM, reduced the amplitude of compound action potentials (3.9 +/- 0.6% to 47.9 +/- 2.4%) and produced at high frequency dependent (phasic) and independent (tonic) conduction block. Lamotrigine extended the peak time of the compound action potentials significantly without changing the half falling-time (P < 0.05). Lamotrigine reduced the amplitude of the delayed depolarization, which was more pronounced than that of the amplitude of the compound action potentials in the presence of 4-aminopyridine. With tonic and phasic stimulation, 0.1 to 10 mM of levetiracetam did not alter the amplitude, peak time and half falling time of the compound action potentials. In addition, levetiracetam did not change the amplitude of the delayed depolarization and the area of the compound action potentials following application of 4-aminopyridine. These results indicate that lamotrigine produces a powerful tonic block with delayed depolarization, whereas it produces a weaker phasic block in rat sciatic nerve. Levetiracetam has no effect on peripheral nerve conduction even at high concentrations. These results may have the relevance to our understanding of the peripheral effects of lamotrigine and levetiracetam. (c) 2006 Elsevier B.V. All rights reserved.