The proximal stumps of five rat sciatic fibers, transected 72 hr earlier, were reconstructed on the basis of morphometry in subserial electron micrographs of isolated fibers. Three fibers showed extensive axon sprouting; 2 had no sprouts but were excessively swollen. The total volume of axoplasm in the axon swellings approximated the volume of axoplasm in all sprouts of any given fiber. Axonal swelling therefore may ensur when sprouting is frustrated. Axon sprouts originated mostly at nodes from where they descended or ascended along the fiber, running within its lamina basalis. Sprouting began soon after injury, usually within the first day. Counts of microtubules showed an approximately 10-fold increase in the total number of tubules per fiber toward the injured end. Schwann cells showed asymmetric hypertrophy, having distinctly more cytoplasm distally than proximally to the nucleus. The increase in Schwannian cytoplasm occurred roughly pari-passu with the increases in axoplasm. Hypertrophy of Schwann cells was associated with cytoplasmic islands or strands having an extremely variable content of organelles. Such islands of Schwannian cytoplasm may be confused with axon sprouts. Retraction of the myelin sheaths at nodes results in fiber profiles suggestive of partial demyelination. Retraction of nodal pseudopodia produces redundant loops of lamina basalis. Migratory cells are seen outside the fibers or underneath their lamina basalis having a preference for nodal regions or for the fiber stump. They behave differently toward axon or myelin: they encompass axon sprouts as do immature Schwann cells; simultaneously the same cell may invade myelin sheaths like a macrophage. Other curious overlaps of degenerative and regenerative phenomena were noted, including an axon sprout tunneling through the lumen of the sequester of the myelin sheath of its parent axon.