The acute "axonal" form of Guillain-Barre syndrome is characterized by rapid progression to severe widespread paralysis and respiratory dependence within 2-5 days of the onset of weakness with very poor and delayed recovery. In 3 cases studied within the first 7 days, the maximum thenar, hypothenar, tibialis anterior, and extensor digitorum brevis "M" potentials were either very reduced in size or absent in response to stimulation at the usual most distal sites of stimulation at the wrist, fibular head, and/or ankle. In the latter instances, advancing the site of stimulation closer to the motor point often evoked an M response. Furthermore, continued distal advance of the site of stimulation evoked progressively larger sized M potentials. Over succeeding days even these very distally evoked M potentials disappeared. Maximum conduction velocities in motor nerve fibers just prior to total loss of excitability were often very reduced. The pattern in these cases is most consistent with progressive loss of excitability and conduction in nerve fibers undergoing axonal degeneration, although coexisting primary demyelination in the terminal segment could not be excluded as the basis for the sometimes very slowed conduction velocities.