We carried out serial nerve conduction studies in a patient with tetrodotoxication caused by ingesting pufferfish. Conduction velocities and amplitudes of muscle and sensory nerve action potentials were equally affected. Neither temporal dispersion nor focal conduction block occurred. The proximal (F wave) motor latencies were also prolonged. These abnormalities rapidly improved in parallel with clinical recovery and with the decrease in the urinary excretion of tetrodotoxin (TTX). These results indicate that TTX equally and reversibly affects myelinated nerve fibers throughout the entire length of the axon by lowering the conductance of sodium currents at nodes of Ranvier.