Introduction. Depression and impulsivity has been associated to a dysfunction of serotonergic system and are treated with serotonin reuptake inhibitors (SSRI). In this work we study differential effects of fluoxetine on impulsivity and depressive symptoms in borderline personality disorders patients. Subjects and method. 38 borderline personality disorders patients without Axis I disorder were treated by seven weeks with flexible doses of fluoxetine. Patients were evaluated with BPRS, GAF, Hamilton Depression Rating Scale and an impulsivity scale. Temporal variations of symptomatology in high and low impulsive patients were compared. Results. Global symptomatology, depressive symptoms and impulsivity diminished. High impulsive patients present reduction in impulsivity in the first two weeks of treatment. Likewise, low impulsive patients had great reduction of depressive symptoms since the third week of treatment. Conclusions. Fluoxetine has a differential effect on impulsivity and depressive symptoms in borderline personality disorders. These results are similar to the reports of other authors in this area and suggest that ISRS act by different neurobiological mechanisms on impulsivity and depression