A morphometric study of atrophy of nerve fibers was made in the ventral and dorsal roots of rats, four to 48 weeks (wk) after transection of their sciatic nerves and with regeneration prevented. The pathophysiological events of fiber atrophy may be summarized as follows: wasting of the axon caliber coincides with a loss of neurofilaments with relative preservation of microtubules. This leads to non-circularity of the fiber, evident from four wk on. The caliber of the circular profile is also reduced. Adaptive changes in myelin sheath structure follow. Thinner fibers with relatively thick sheaths are first detectable after eight wk and prominent after 24 to 48 wk. This change may indicate passive slippage of the sheath, but sheath remodelling in adjustment to a changed internodal geometry appears more likely.