There is evidence to suggest that oxytocin is effective in stabilizing mood in humans. Lower plasma oxytocin levels have been reported in patients with major depression. The objective of this study was to investigate serum oxytocin levels during manic and depressive episodes and in the remission period in patients with bipolar disorder. Twenty-two patients in manic episode, 21 in depressive episode, and 24 in remission at the initial phase, ranging from 18 to 65 years of age, who were diagnosed with BD Type I and 24 healthy individuals were included in this study. Blood samples were collected from subjects in the morning at the beginning of the study. A second blood sampling was obtained from manic and depressive patients after response to treatment. MANCOVA was performed to compare the oxytocin values of the groups. The serum oxytocin levels of patients in manic episode were statistically significantly higher than those of the depressive episode and remission groups and of the healthy subjects. The serum oxytocin levels of patients in the depressive episode group and in the remission group were statistically significantly higher than those of the control group. The serum oxytocin levels of the manic episode and depressive episode patients after response to treatment were statistically significantly higher than those of the control group, and there was no statistically significant difference between the patient groups in serum oxytocin levels. The higher oxytocin levels observed in patient groups, compared to the controls, before and after response to treatment suggest that oxytocin may be a trait marker in BD. (C) 2013 Elsevier Ltd. All rights reserved.