Nitric oxide (NO) has been implicated to play a role in the pathogenesis of many neuropsychiatric disorders. NO level was found high in acute manic inpatients. In this study, we aimed to assess NO level and activity of the antioxidant enzyme, superoxide dismutase (SOD), in euthymic bipolar patients. Twenty-seven patients with bipolar disorder (BD) in euthymic phase, and 20 healthy volunteers were included in this study. A semi-structured form was used to note social, demographic and clinical parameters of the patients. NO level and SOD activity were studied in the serum samples obtained from the patients and controls. The mean serum NO level in BD was significantly higher than in controls. Mean serum SOD activity was found to be elevated in patients with BD compared to controls. Total number of the manic episodes correlated with NO levels, but not with SOD activity. In conclusion, the number of manic episodes is positively associated with NO levels. NO and SOD appear to have a pathophysiological role in BD, especially in Type I euthymic phase, and may be considered an available trait marker for BD.