Introduction: Psychiatric disorders could be evaluated in terms of behavioral activation and inhibition systems. Dysregulation of these systems may lead to development of manic or depressive episodes in patients with mood disorders. This study aimed to identify Behavioral Approach System (BAS) and Behavioral Inhibition System (BIS) hypersensitivity as the functional brain system behaviors in patients with major depressive disorder and bipolar mood disorder I, compared to healthy individuals. 
Methods: This case-control study was conducted in Razi Psychiatric Hospital, a mental health referral center in Northwest of Iran. The study consisted of two groups of patients, one with major depressive and the other with bipolar mood disorders and one healthy group. Each group had 40 patients (20 men and 20 women). The study data were collected through BIS and BAS questionnaire, Beck Depression Inventory (BDI-II), Young Mania Rating Scale (YMRS). The obtained data were analyzed by SPSS version 18. 
Results: The findings showed a significant negative correlation between BIS, BAS and BAS subscales with the severity of depression and positive correlation with mania symptoms (P<0.05). 
Conclusion: BAS and BIS dysregulations may predispose people to mood disorder symptoms. BAS is hyperactive during manic phase and may predict the symptom severity of bipolar mood disorder.