To elucidate whether an inducible nitric oxide synthase (iNOS) inhibitor, aminoguanidine (AMG), affects the expression of iNOS in the spinal cords of rats with experimental autoimmune encephalomyelitis (EAE), we induced EAE in Lewis rats, and treated EAE rats with AMG. AMG (200mg/kg administered intraperitoneally from day 0 to day 7 post-immunization) significantly reduced the clinical severity of EAE paralysis. AMG, however did not prevent the occurrence of EAE. Western blot analysis showed that iNOS expression was significantly reduced in the spinal cords of rats with EAE treated with AMG compared with rats treated with the vehicle. This finding supports the conclusion that the production of nitric oxide by iNOS plays an important role in the induction of EAE. The corollary is that the amelioration of EAE paralysis by the treatment with AMG is associated with the suppression of iNOS expression in the target organ i.e. the spinal cord.