[en] BACKGROUND: A number of challenge studies have reported abnormalities of serotonergic function in borderline personality disorder (BPD). There are, however, problems with the pharmacological probes used in these studies since fenfluramine and m-CPP are not only serotonergic agents but also induce release of catecholamines, particularly dopamine. Therefore, we tested whether subjects with BPD showed a blunted prolactin (PRL) response to flesinoxan, a highly potent and selective 5-HT1A agonist. METHODS: Flesinoxan challenge test was carried out in 20 BPD in-patients and 20 healthy controls matched for gender but not for age. Since 16 BPD in-patients exhibited major depressive co-morbidity, a group of 20 depressed in-patients matched for gender but not for age was also included. RESULTS: BPD in-patients exhibited blunted PRL responses as compared to controls, whereas depressed in-patients did not differ from controls. Moreover, PRL responses were lower among BPD in-patients than among depressed in-patients. Among the BPD in-patients, PRL responses to flesinoxan were lower in patients with past history of suicide attempts (N = 8) than in those with a negative history. CONCLUSIONS: The results show major involvement of serotonergic function in BPD and are consistent with previous studies linking lower serotonergic activity with impulsivity. More particularly, our data suggest that BPD is characterized by lower 5-HT1A receptor sensitivity. Moreover, the data support the involvement of 5-HT1A activity in suicidal behaviour. However, this conclusion is limited because other hormonal responses such as ACTH and cortisol were not assessed, and because BPD was assessed by a self-report questionnaire and not a structured clinical interview.