Guillain-Barré syndrome (GBS) includes demyelinating and axonal subtypes with different immunopathologic mechanisms. In acute inflammatory demyelinating polyradiculoneuropathy, segmental demyelination and conduction block are the pathological and electrophysiological correlates of muscle weakness. Slowed conductions and increased temporal dispersion of motor responses are more characteristic of the remyelinative phase and do not affect muscle power. In acute motor axonal neuropathy, muscle weakness has been correlated with an antibody-mediated primary axonal degeneration. Conduction block that recovers without development of increased temporal dispersion or other demyelinating features, however, has been described in some patients with antiganglioside antibodies and related to a physiologic conduction block at the axolemma of the Ranvier node. Severity of axonal damage induced by antiganglioside antibodies may vary from reversible functional impairment of nodal axolemma to complete axonal damage with subsequent Wallerian degeneration. In early GBS, current electrophysiologic criteria are unable to distinguish with certainty different subtypes. Serial electrophysiologic studies are mandatory for identification of GBS subtypes and to elucidate the pathophysiologic mechanisms of muscle weakness among demyelination, axonal degeneration and physiologic conduction block.