Dissociated dorsal root ganglion cells from human fetuses were exposed to 2,5-hexanedione (2,5-HD) for 2 weeks. Morphological changes induced by 2,5-HD consisted in focal neurofilament (NF)-containing enlargements preferentially located in distal, preterminal regions of unmyelinated fibers. Tangles of NF were also observed in the perikarya of nerve cells. Morphometric analysis disclosed that the cross-sectional areas of the 2,5-HD treated axons were 30% smaller than those of control axons. This alteration was associated with reduction of number of NF per unit area. These findings demonstrate that 2,5-HD treatment induces a generalized disorganization of neuronal and axonal NF responsible for focal enlargements as well as atrophic changes of unmyelinated fibers.