Parkinson's disease (PD) is characterized by severe movement disorders. Excessive production of reactive oxygen species has been implicated as a possible mediator of neurodegenerative changes in nigrostriatal neurons in PD. Nitric oxide (NO) has received substantial attention among pathophysiological factors underlying glutamate neurotoxicity in neurodgenerative disorders including PD. PD is more prevalent in men than in women by an approximate 3:2 ratio; the nitrergic systems are also more active in males than females. In this study, NO2- +NO3- (stable metabolites of NO) levels were determined in sera of PD patients (n=20) and control subjects (n=16). NO2- +NO3- Levels were higher in PD patients compared to controls, and in males compared to females. Furthermore, there was a correlation between age and NO metabolites in females but not in males. These data support a role for NO in PD and provide further evidence for sex differences in NO activity that may underlie neurodegenerative disorders.