The relationship between human consciousness and oxygen saturation (rSO(2)) in cerebral tissue under high +Gz stress was non-invasively monitored by near-infrared multiple wavelength spectroscopy (NIRS). We studied the drop in rSO(2) levels in human subjects during exposure to various head-to-foot acceleration (+Gz) profiles. These profiles included sustained +Gz plateaus and repeated short duration +Gz pulses of varying duration. The end point in this study was +Gz-induced loss of consciousness (G-LOC). The rSO(2) levels under normal (asymptomatic), almost loss of consciousness (A-LOC) and G-LOC conditions were recorded. Correlations among decrease in rSO(2), +Gz pulse duration, +Gz stress level and incapacitation time (ICAP) after G-LOC were also investigated. It was found that once rSO(2) fell to a certain level, G-LOC occurred. This threshold was repeatable and independent of the +Gz level or duration. It was also observed that the total ICAP after G-LOC was dependent on the length of time that rSO(2) remained below the G-LOC threshold level, i.e. the longer the rSO(2) level remained below the G-LOC induction level, the longer the subject remained unconscious. These results may prove to be useful in designing closed loop control systems for personal protective gear for pilots of high performance aircraft.