Kittens, aged 7–9 weeks, were killed in groups of four at survival periods of 0 (unoperated controls), 1, 2, 3, 5, 10, 15, and 28 days following left brachial plexotomy. [ 3H]Leucine was administered intraperitoneally, 10 μCi/g body wt, 0.25, 0.5, 2, or 16 hr before sacrifice. Qualitative and quantitative histologic and autoradiographic observations were made. On the axotomized side, chromatolysis was qualitatively evident in 97% of C-8 motoneurons 10 days postoperatively. Cellular enlargement was highly variable. Nuclear atrophy was frequent 10–15 days after surgery. Regenerating axons penetrated the neuroma 5 days after plexotomy and reached the ulnar nerve at the level of the olecranon at 28 days and neuromuscular junctions of flexor carpi ulnaris at 60 days. Axotomized cervical motoneurons from kittens sacrificed 0.25 hr after administration of [ 3H]leucine on the 10th or 15th postoperative day exhibited a relative decrease in the uptake of labeled amino acid. A similar decrease occurred in the 28-day survival sacrificed 0.50 hr after injection of labeled amino acid. In contrast when animals were killed at the 2.0 hr postinjection period, a relative increase in the somal accumulation of tritiated leucine occurred 2 through 28 days post-operatively. A comparable increase obtained 16.0 hr after isotope injection and 2–10 days after plexotomy. The differences in the direction of change observed at short (0.25 and 0.5 hr) and long (2.0 and 16.0 hr) labeling intervals suggest that alterations in accumulation of radiolabeled amino acid by axotomized somas may be related to changes in the types of proteins synthesized or to alterations in the rates of export and/or turnover of cytoplasmic proteins. The increased cytoplasmic radioactivity observed after 2.0- and 16.0-hr labeling intervals preceded light microscopic evidence of axon regeneration and was not dependent on reestablishment of peripheral contacts.