The results of unilateral ventral root avulsions at the cervical 7 (C7) level were compared using light microscopy with unilateral transactions of the ventral root C7 at the surface of the cat spinal cord after survival times ranging from one to 90 days. Sections were stained for Nissl, acetylcholinesterase, neurofilaments, glial fibrillary acidic protein, and B-50/GA P43. A distinct difference in neuronal reaction was found between the effects of avulsion and transection of ventral root C7. After avulsion, changes in the Nissl-and acetylcholinesterase-stained sections and the sections stained with antibodies against neurofilaments remained present up to 90 days, although the number of neurons in the ventral horn on the avulsed side had significantly decreased at 60 and 90 days. This was in contrast to the transection where the changes in the stainings tended to return to the normal situation between 30 and 60 days and showed the normal pattern 90 days after transection. No decrease in the number of neurons was found. Both lesions were situated in the transitional region of the nerve roots. After avulsion axons break in the white matter of the CNS, while the axons were transected in the PNS compartment of the CNS-PNS transitional region. After avulsion, not only were the axons disconnected, as in transection, but the connections of recurrent motor axon collaterals probably were damaged. This may explain the different reaction after avulsion and transection.