Objective: Function of nitric oxide (NO) and its intra-system competitive inhibitor of NO, asymmetric dimethylarginine (ADMA) has been investigated in mood disorders, schizophrenia, autism, obsessive compulsive disorder and Alzheirmer disorder and significant results were obtained. The mechanism of action in mood stabilizators is still unclear despite many years of studies. The parameters of oxidative stres were shown to decreased upon antimanic treatment in bipolar disorder (BD). The lithium may be suggested to produce neuroprotective effects against oxidative damage by regulatory effects towards various targets. The effects of lithium on nitric oxide synthase were researched in labarotory studies. In this study, two tubes of plasma were takenfrom the patients for determination of NO and ADMA during manic and eutdhymic phases. Methods: Seventeen patients who presented to the psychiatry or emergency departments of Farabi Hospital of Karadeniz Technical University and were diagnosed with BAD-I manic episode and followed with lithum were included into the study. The Structured Clinical Interview for DSM-IV Axis I Disorders (SCID-I), Hamilton Depression Rating Scale (HAM-D), Young Mania Rating Scale (YMRS), Montgomery Asberg Depression Rating Scale (MADRS), were applied. For determination of NO and ADMA levels, two tubes of plasma were collected during the periods of mania and euthymia in the patient group. Results: The level of ADMA were significantly lower while the level of NO were higher in the posttreatment group than the pretreatment group. Conclusion: In this study, the level of NO were increased, the level of ADMA were dicreased in response to lithium treatment, the therapeutic effect of lityum may contribute this mechanism.