Nitric oxide (NO) has been implicated in immune mediated cellular cytotoxicity and inflammatory processes including multiple sclerosis (MS). We aimed to assess NO production in MS patients and to delineate its involvement in different stages. The stable end-products of NO; nitrite(NO2-) and nitrate(NO3-) were analysed both in serum and CSF (cerebrospinal fluid) of patients with MS and non-inflammatory neurological diseases. Nitrite levels were quantified by calorimetric assay based on the Griess reaction. Nitrate levels were examined spectrophotometrically. MS patients exhibited significantly increased Serum and CSF levels of NO2- + NO3- compared with the control subjects. CSF NO2- + NO3- levels were raised significantly in MS patients with both relapsing remitting (RR) and secondary progressive (SP) course. There was no significant difference between RR and SP MS patients with regard to NO metabolites. No significant correlation was found between NO metabolites and disability score, disease progression index, MRI (magnetic resonance imaging) activity and development of cortical atrophy on MRI. This study provides further evidence for excessive NO production both in CSF and peripheral blood of MS patients. Excessive CSF NO2- + NO3- levels being more increased than the levels in sera supports pathological inflammatory process within CNS (central nervous system) in both stages of MS. Another implication for the role of NO and INOS inhibitors in the treatment of MS patients with both RR and SP courses was also suggested. (C) 2001 Elsevier Science B.V. All rights reserved.