Rats were given isoniazid either in a single large dose or continuously in drinking water and killed 5--105 days later. The distribution of degenerating fibres in various nerves (sensory and mixed) and in various sites along nerves and spinal roots was studied by light and electron microscopy. It was found that sensory nerves tended to be less affected than motor, and degeneration was more proximal in the latter. It was concluded that the pattern of degeneration and regeneration supported the view that the initial metabolic lesion is in the axon, rather than in the cell body.