It has been suggested that nitric oxide (NO) could be implicated in the pathogenesis of multiple sclerosis (MS). Recently, two groups reported increased cerebrospinal fluid (CSF) nitrate levels (oxidation product that provides an indirect estimation of NO) in MS patients. However, another group did not confirm these findings. We studied the CSF and plasma levels of nitrate with a kinetic cadmium reduction method in 11 MS patients and 25 matched controls. The CSF nitrate levels and the CSF/plasma nitrate ratio did not differ significantly between the two study groups. Plasma nitrate levels were nearly significantly lower in MS patients. CSF and plasma nitrate levels did not correlate with age at onset and duration of the disease in the patient group. These data suggest that measurement of CSF levels of nitrate is not a marker of the activity of MS.