Background: Anorexia nervosa is a debilitating eating disorder with profound biological and psychological consequences. Recent evidence implicates nitric oxide abnormalities in the pathogenesis of anorexia nervosa. Objective: To investigate nitric oxide bioavailability in red blood cells from adolescents with anorexia nervosa and its association with the arginase pathway, redox state and systemic amino acid profile. Methodology: A case-control study of plasma amino acid levels, nitric oxide synthase and arginase activities, and oxidative stress markers (lipid peroxidation, protein oxidation, reactive oxygen species production and superoxide dismutase activity) in red blood cells from volunteer adolescents with anorexia nervosa (n=10) recruited at the Center for Studies on Adolescent Health – State University of Rio de Janeiro, and healthy controls (n=10), age-matched. The Ethical Committee of the State University of Rio de Janeiro approved this work (CEP -1485), and informed consent was obtained from each of the patients. The student t test was used for statistical analysis and statistic difference was considered when p<0.05. Results: Nitric oxide synthase activity was impaired in red blood cells (controls: 8.22 ± 1.45 vs patients: 3.88 ± 0.46 pmol/108 cells/min) in the presence of increased arginase activity in anorexia nervosa (controls: 0.02 ± 0.01 vs patients: 0.09 ± 0.03 pmol urea/mg protein/2h). Asymmetric dimethylarginine, an endogenous inhibitor of nitric oxide synthase, was not affected, whereas L-lysine (controls: 160 ± 10 vs patients: 130 ± 7 μM/L) and L-ornithine (controls: 93 ± 9 vs patients: 64 ± 6 μM/L) were reduced in plasma from patients with anorexia nervosa compared to controls. No oxidative damage or increased reactive oxygen species production was observed in anorexia nervosa. On the other hand, diminished anti-oxidant protection by superoxide dismutase was present in red blood cells from patients with anorexia nervosa (controls: 15260 ± 2803 vs patients: 4791 ± 678.4 U of SOD/g of haemoglobin). Conclusions: Our findings provide the first evidence of reduced nitric oxide synthesis associated with diminished antioxidant defence and arginase activation in red blood cells, which may contribute to impaired vascular nitric oxide bioavailability in anorexia nervosa.