TO investigate the sites of nitric oxide synthase (NOS) expression after a spinal cord (SC) injury, NADPH-d diaphorase histochemistry was performed in the SC of adult rats sacrificed at different times from 1h to 90 days after both SC contusion or transection. NOS could first be seen 12h after injury in axonal swellings (AS) (club shaped structures at the tip of damage axons, associated with tissue destruction). NOS expression reached a maximum 3 days after injury, and gradually disappeared after 7 days. Finally, AS collapsed leaving behind micro-cysts. NOS expression and the consequent production of nitric oxide could be involved in the pathophysiology of the secondary damage, and/or could reflect a failed attempt for axonal regeneration