Crush of sciatic nerve in rabbits supported by morphologic and electrophysiologic data was used to evaluate the effects of ACTH4-10 on nerve fiber regeneration. Treated animals showed a statistically significant higher regeneration rate than did control rabbits. Fiber density and mean diameters of myelinated nerve fibers were measured in semithin nerve sections at 1 and 3 cm distal from the crushed point at three different time points. Nerve fiber density results were higher in ACTH4-10-than in vehicle-treated rabbits. This difference showed an unequivocal trend and attained a statistically significant level in the sections 3 cm distal from the crush. ACTH4-10 seemed therefore to have a beneficial effect on nerve fiber regeneration.