Abnormal glutamate metabolism is implied in the pathogenesis of Amyotrophic Lateral Sclerosis (ALS) and cerebrospinal fluid (CSF) glutamate levels appear to be elevated. Since nitric oxide (NO) inhibits glutamate transport, excessive amounts of nitric oxide could underlie the glutamate induced neurotoxicity in ALS. Stable metabolites of NO (NO2- + NO3-) levels were determined in serum and CSF of sporadic ALS patients and control subjects. NO2- + NO3- levels were higher in ALS, in males and in serum samples compared to controls, females and CSF, respectively. Furthermore, while the difference between serum and CSF NO2- + NO3- levels was significant in males (higher in serum) no such difference was observed in females. Our results suggest that nitric oxide may be involved in the pathogenesis of ALS directly or indirectly and in a sexually dimorphic manner.