To assess the therapeutic potential of brain-derived neurotrophic factor (BDNF) in clinics, we extensively investigated the effects of BDNF on adult motor neurons in a rat spinal root avulsion model. Intrathecal administration of BDNF immediately after the spinal root avulsion greatly protected against the motor neuron cell death. BDNF also showed a protective effect on the atrophy of soma and on the reduction of transmitter-related enzymes such as choline acetyl transferase and acetylcholine esterase. Very interestingly, BDNF induced axonal outgrowth of severely damaged motor neurons at the avulsion site. The BDNF administration following 2-week treatment with phosphate-buffered saline after avulsion prevented further augmentation of cell death and reversed cholinergic transmitter-related enzyme deficiency. BDNF was demonstrated to possess a wide variety of biological effects on survival, soma size, cholinergic enzymes, and axonal outgrowth of adult motor neurons. These results provide a rationale for BDNF treatment in motor neuron diseases such as spinal cord injury and amyotrophic lateral sclerosis.