Electrophysiological studies of sectioned crayfish motor axons of the abdominal flexor muscle have demonstrated that the distal stumps, isolated from their central connections for up to several months, are remarkably resistant to degeneration, as previously shown in the crayfish claw opener preparation. These distal stumps remained electrically excitable and intracellular recording showed that their neuromuscular junctional properties were indistinguishable from normal ones in such properties as waveform and amplitudes of the evoked junctional potentials. Furthermore, repetitive stimulation of the distal stumps evoked jp's which showed facilitation as in normal preparations. When degeneration finally occurred, electrical stimulation of the nerve no longer evoked jp's, and no progressive deterioration of junctional properties was observed.Regeneration ultimately occurred in these nerves but required at least three weeks or more. The electrical properties of these regenerated junctions were not different from normal ones.The evidence from time-course and correlated histological studies of degeneration and regeneration suggest that regeneration of these crayfish nerves occurs by axonal fusion rather than by outgrowth of an entirely new distal apparatus from centrally located cell bodies, as is the mechanism in vertebrates and some invertebrates.