Recent research suggests that emotional dysfunction in psychiatric disorders can be reflected in autonomic abnormalities. The present study examines sympathetic and parasympathetic autonomic nervous system activity in individuals with Borderline Personality Disorder (BPD) before, during, and following a social stressor task. Data were obtained from an analogue sample of participants screening positive for BPD (n = 12) and healthy controls (n = 28). In general, BPD participants exhibited increased sympathetic activity (indexed by Cardiac Sympathetic Index, CSI; Toichi et al., 1997) and decreased parasympathetic activity (indexed by Respiratory Sinus Arrhythmia, RSA) compared to controls. During the stressful task, BPD and control participants exhibited different trajectories of sympathetic activation: estimates of sympathetic activity increased for BPD participants and decreased for controls. Furthermore, BPD participants reported the task (but not baseline or recovery phases) to be more frustrating than controls. Findings are interpreted in the context of Polyvagal theory. (Contains 1 table and 3 figures.)