Some amino acids are involved in the biosynthesis of nitric oxide (NO), which has a physiological and pathophysiological role. To study NO biosynthesis, we compared arginine and citrulline levels in the cerebrospinal fluid (CSF) from patients with infectious and/or inflammatory processes within the central nervous system (CNS), with those from patients without those disorders. Arginine concentration was not significantly different between the groups (P = 0.115), whereas citrulline was significantly elevated in the first group (P = 0.020). We propose a simple chromatographic method to estimate NO biosynthesis ex vivo within the CNS, that may be applicable for the study of neurodegenerative and psychiatric diseases such as Parkinson's disease and schizophrenia.