AbstractAxons in tissue culture retract and shorten if their tips are detached from the substrate. The shortening reaction of the axon involves contractile forces that also arise during normal axonal motility, elongation, and retraction. We studied shortening in axonal segments isolated from their parent axons by transecting the axon between the growth cone and the most distal point of adhesion to the substrate. Within 15–20 minutes after transection, an isolated axonal segment shortened and pulled its tail end toward the growth cone. During the shortening process, long sinusoidal bends arose along the axon. The identical shortening reaction occurs without transection, when the axon tip is detached from the substrate. Pharmacological studies with inhibitors of glycolysis indicate that the shortening mechanisms utilize metabolic energy, presumably ATP. The rate of sinusoidal shortening is similar to both the rate of polymer translocation in the axon by slow axonal transport and the rate of normal axonal elongation. Taxol inhibits the shortening reaction with a similar dose dependence to its inhibition of axonal growth. Together, all these observations suggest that the same basic intracellular motility mechanisms are involved in normal axonal growth, in slow axonal transport, and in the shortening reaction: the intracellular dynamic system that utilizes ATP to generate longitudinal movements of polymers within the axon may be the same mechanism underlying both the retraction and the elongation of the axon.