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The aim of this study was to analyze the relationships between quetiapine and N-desalkylquetiapine plasma levels and clinical improvement, particularly, in regard to depressive and anxious symptoms and to hostility.This was a prospective observational study that involved 37 outpatients diagnosed as having bipolar disorder I or II. All the patients were observed during a clinical acute and postacute phase. Patients were prescribed 50–800 mg of quetiapine. Patients were evaluated at baseline, after 15 days and after 3 months using the Brief Psychiatry Rating Scale with particular reference to the dimensions of depression, anxiety, and hostility. The plasma concentrations of quetiapine and N-desalkylquetiapine were determined after 3 months using blood samples taken at steady state.There was a significant relationship between the N-desalkylquetiapine/quetiapine ratio and the improvement in the depression dimension, and there was not a significant relationship between the N-desalkylquetiapine/quetiapine ratio and anxiety and hostility improvement. Quetiapine treatment was well tolerated, and there were no extrapyramidal, anticholinergic, or other side effects to note. There was no relationship between plasma quetiapine or N-desalkylquetiapine concentrations and side effects.Our findings confirm the efficacy of quetiapine on depressive symptoms, and the available data support that quetiapine's antidepressant activity is mediated by the active metabolite norquetiapine, and it exemplifies the case of an active metabolite that can make a drug like quetiapine originally introduced as an antipsychotic a useful antidepressant agent.