Ten healthy human volunteers were subjected to progressive lower body negative pressure (LBNP) to the onset of cardiovascular collapse to compare the response of noninvasively determined, skin and fat corrected deep muscle oxygen saturation (SmO₂) and pH to standard hemodynamic parameters for early detection of imminent hemodynamic instability. Muscle SmO₂ and pH were determined with a novel near infrared spectroscopic (NIRS) technique. Heart rate (HR) was measured continuously via ECG, and arterial blood pressure (BP) and stroke volume (SV) were obtained noninvasively via Finometer^TM and impedance cardiography on a beat-to-beat basis. SmO₂ and SV were significantly decreased during the first LBNP level (-15 mm Hg) whereas HR and BP were late indicators of impending cardiovascular collapse. SmO₂ declined in parallel with SV and inversely with total peripheral resistance suggesting, in this model, that SmO₂ is an early indicator of a reduction in oxygen delivery through vasoconstriction. Muscle pH decreased later, suggesting an imbalance between delivery and demand. Spectroscopic determination of SmO₂ is noninvasive and continuous providing an early indication of impending cardiovascular collapse resulting from progressive reduction in central blood volume.