Anorexia nervosa (AN) is an eating disorder that leads to a marked loss of weight and important clinical complications. Nitric oxide (NO) is a gas involved in vascular homeostasis. Its synthesis occurs through the conversion of the semi-essential cationic amino acid L-arginine into L-citrulline and NO, by the action of the enzyme NO synthase (NOS)¹. Arginase is an enzyme involved in urea cycle that competes with NOS by L-arginine. Bioavailability of NO depends not only on its production, but also on its degradation by reactive oxygen species (ROS). The aim of this study was to investigate the NO production, urea cycle and oxidative stress markers as a possible indicative of bioavailability of NO in red blood cells from patients with AN. Eight female patients with AN from the Núcleo de Estudos e Saúde dos Adolescentes (NESA) / UERJ and eight age and sex-matched healthy volunteers participated in this study. The Pedro Ernesto Hospital Ethical Committee approved this study, and informed consent was obtained from each participant. Basal NOS activity was determined by the conversion of L-[³H]-arginine into L-[³H]-citrulline. Arginase activity in erythrocytes was determined by the conversion of L-[¹⁴C]-arginine to [¹⁴C]-urea. As an index of lipid peroxidation, the thiobarbituric acid-reactive substance formation (TBARS) was evaluated during an acid-heating reaction, as previously described by Draper et al.². Superoxide dismutase (SOD) activity was assayed by measuring the inhibition of adrenaline auto-oxidation at 480 nm. Statistical significance was assessed using Student t-Test, with p <0.05. The results showed a diminished NOS activity in patients with AN compared with controls (3.8 ± 0.4 vs. 8.2 ± 1.4 pmol/10⁸cells/min; n=8). An increased activity of arginase was demonstrated in patients with AN in relation to controls (0.09 ± 0.02 vs. 0.01 ± 0.00 pmol urea/mg of protein/2h; n=8). An unchanged formation of TBARS (0.001 ± 0.000 vs. 0.003 ± 0.001 nMol/mg of protein; n=8) associated with elevated SOD activity (0.42 ± 0.07 vs. 0.12 ± 0.02 U/mg of protein; n=8) was found in AN patients in comparison to controls Our results demonstrated that in red blood cells from patients with AN the NO production is diminished and it can be in part attributed to an increased arginase activity. The activity of antioxidant enzyme SOD was elevated in AN while TBARS concentration was unaffected. The final correlation between oxidative stress and NO bioavailability needs to be further investigated.