Orthostatic reflexes were examined at 375 m and, after 60 minutes of exposure in a hypobaric chamber at 3,660 m using a 20-minute seventy-degree head-up tilt (HUT) test. Mean arterial blood pressure, RR-interval (RRI), and mean cerebral blood flow velocity (MFV) were examined with coarse-graining spectral analysis. Of 14 subjects, seven at 375 m and 12 at 3,660 m were presyncopal. Immediately upon arrival to high altitude, breathing frequency and MFV increased and end-tidal PCO₂ (PETCO₂), RRI, RRI complexity and the parasympathetic nervous system (PNS) indicator decreased. MFV was similar in HUT at both altitudes. The sympathetic nervous system (SNS) indicator increased with tilt at 3,660 m while PNS indicator decreased with tilt at both altitudes. Multiple regression analysis of supine variables from either 375 m or 3,660 m and the time to presyncope at 3,660 m indicated that, after one hour of exposure, increased presyncope at altitude was the result of: 1) ineffective peripheral vasoconstriction, despite increased cardiac SNS activity with HUT; and, 2) insufficient cerebral perfusion due to cerebral vasoconstriction as the result of hypoxic hyperventilation-induced hypocapnia.