Background: Chronic orthostatic intolerance(COI) is a common and disabling aut onomic syndrome of unclear pathophysiology. We tested the hypothesis that barore flex and autonomic responses to graded lower body suction (LBNP, up to 40mmHg) could be altered in COI patients. Methods: Electrocardiogram (ECG), non invasi ve arterial blood pressure and respiratory activity were measured during progres sive LBNP (seven patients and seven volunteers). Lumped arterial barore flex se nsitivity (a index), and its arterial and cardiopulmonary components,were assess ed by multivariate closed loop analysis of RR interval and systolic arterial pr essure(SAP) spontaneous variabilities and respiration. Monovariate spectral anal ysis of RR interval and SAP variability provided markers of autonomic regulation of the sinoatrial(SA) node and of vascular sympathetic modulation. Results: Sim ilar reductions in overall and cardiopulmonary baroreflex gain were observed in both groups in response to graded LBNP. In contrast, only controls demonstrated a selective increase in arterial baroreflex sensitivity, at low grade LBNP. Cle ar increases in the low frequency component of RR interval variability (LFRR) [ and decreases in the high frequency component of RR interval variability (HFRR) , both in normalized units]were observed in controls with graded LBNP, while in s ignificant changes occurred in COI patients, who showed, conversely, exaggerated sympathetic vasomotor responses [as assessed by the low frequency component of SAP variability(LFSAP)]. Conclusions: Patients with chronic orthostatic intoler a nce show distinct signs of altered baroreflex and autonomic regulation of the SA node and of the vasculature in response to graded LBNP. Background: Chronic orthostatic intolerance (COI) is a common and disabling aut onomic syndrome of unclear pathophysiology. We tested the hypothesis that barore flex and autonomic responses to graded lower body suction (LBNP, up to 40mmHg) could be altered in COI patients. Methods : Electrocardiogram (ECG), non invasi ve arterial blood pressure and respiratory activity were measured during progres sive LBNP (seven patients and seven volunteers). Lumped arterial barore flex sensitivity (a index), and its arterial and cardiopulmonary components, were assess ed by multivariate closed loop analysis of RR interval and systolic arterial pr essure (SAP) spontaneous variabilities and respiration. Monovariate spectral anal ysis of RR interval and SAP variability provided markers of autonomic regulation of the sinoatrial (SA) node and of vascular sympathetic modulation. : Sim ilar reductions in overall and cardiopulmonary baroreflex gain were observed in both groups in response to graded LBNP Cleav increases in the low frequency component of RR interval variability (LFRR) [and decreases in the high frequency component of RR interval variability (HFRR) , both in normalized units] were observed in controls with graded LBNP, while in ignificant changes occurred in COI patients, who showed, conversely, exaggerated sympathetic vasomotor responses [as assessed by the low frequency component of SAP variability (LFSAP)]. Conclusions : Patients with chronic orthostatic intoler a nce show distinct signs of altered baroreflex and autonomic regulation of the SA node and of the vasculature in response to graded LBNP.