The number of newly formed myelinated nerve fibers was counted in the sciatic and tibial nerves in the rat following sciatic nerve crush. Beginning at days 8 and 14 in the sciatic and tibial nerve, respectively, the number of new myelinated fibers increased steadily and eventually exceeded the original number by 40 and 30%, respectively. This overshoot in nerve fibers was accompanied by the presence of clusters of fibers. Both the overshoot and the clusters disappeared at later stages and returned gradually to normal values within 3 months. Chronic administration of ACTH4-10 to the animals resulted in higher numbers of new myelinated nerve fibers throughout the process of regeneration. This stimulation was most pronounced (a three-fold increase) during the initial stages of regeneration. A higher number of myelinated nerve fibers was also observed in ACTH-treated rats 60 and 96 days after sciatic nerve crush. No changes in diameter of the fibers could be observed. The results are discussed in terms of a stimulation of the number of outgrowing nerve fibers caused by the peptide treatment.