The squid giant axon responded to a transection injury by producing a gradient of cytoplasmic and vesicular changes at the cut end. At the immediate opening of the cut axon the cytoplasm was fragmented and dispersed and the vesicles in this region were in rapid Brownian movement. Approximately 0.1 mm further in, at the site of maximal axonal constriction, the axoplasm was condensed into a compact, constricted mass containing many large vesicles. The axoplasm was normal a few millimetres beyond this constricted, vesiculated end. It appears that transection triggered the transformation of normal axoplasm into a tightly constricted, highly vesiculated structure. This modified axoplasm at the cut end may slow the spread of damage and degeneration by preventing the bulk outflow of axoplasm, by slowing down the loss of intracellular molecules and by slowing down the influx of destructive extracellular ions (like calcium and chloride).