The neuropathy produced by the hexacarbon 2,5-hexanedione (2,5-HD) resembles human and canine inherited giant axonal neuropathy (GAN) in the presence of giant axonal swellings that contain accumulations of neurofilaments. The accumulations are both paranodal and internodal in GAN and 2,5-HD induced neuropathy. Detailed morphometry on the neurofilaments reveals that the changes in human and canine GAN are closely similar and differ from those of 2,5-HD neuropathy, suggesting that the mechanisms underlying the formation of the axonal neurofilamentous accumulations differ between the two conditions. In both human and canine GAN, the neurofilaments are more closely spaced and are of greater diameter than in 2,5-HD neuropathy. The changes in the NF in GAN may be the consequence of flattening of the side-arms of the neurofilaments against the axis of the filaments.