Twelve patients with borderline personality disorder and 15 healthy comparison subjects were challenged with single doses of oral m-chlorophenylpiperazine (m-CPP) and placebo. Following m-CPP, the patients experienced decreased anger and fear. Seven of the 12 patients reported a "spacy," "high," depersonalized/derealized experience following m-CPP, which was confirmed by clinicians' ratings. Compared with the normal male subjects, the male patients with borderline personality disorder had higher cortisol levels and marginally blunted prolactin responses after receiving m-CPP. These results suggest serotonergic dysfunction in borderline personality disorder.