The ability of alpha-MSH to facilitate the recovery of sensorimotor nerve function following crush lesion is restricted to a critical period following such a lesion. This period coincided with the initiation of sprouting and the disappearance of the 150 kD neurofilament protein from the degenerating distal stump of the nerve. Degenerating nerve contains a factor that is active in a bioassay system for MSH. This factor could not be detected in control nerves. The hypothesis is forwarded that a neurotrophic factor known to be present in degenerating nerve stumps is an alpha-MSH-like peptide formed by the breakdown of the 150 kD neurofilament protein.