Cerebral autoregulation (CA) is impaired after prolonged exposure to hypoxia; however, no studies have investigated CA in acute hypoxia. Objective: To assess changes in CA over a 9 hr exposure to hypobaric hypoxia (HX: Pbar = 425 mmHg; 16,000 ft). Methods 26 healthy subjects were studied at baseline (BL: Pbar = 625 mmHg; 5,400 ft) and then following 4 and 9 hrs of HX. Radial artery blood pressure (ABP) and middle cerebral artery blood flow velocity (CBFv) were recorded for 6 min at BL, 4 & 9hr. We assessed ABP to CBFv relationships using cross spectral analysis over 3 regions of physiological interest (B‐waves: 0.008 to 0.05 Hz; M‐waves: 0.05 to 0.15 Hz; R‐waves: 0.15 to 0.50 Hz R‐waves). Increased coherence, gain and/or decreased phase shift are considered indicators of impaired CA. Results During BL, CA resembled a high pass filter (highest gain & lowest phase shift across R‐waves, p<0.01), suggesting intact CA. During HX, spectral density of CBFv, coherence & gain were increased across B‐waves after 4 hr (p<0.05). M‐wave spectral densities of ABP and CBFv increased, but gain was dampened at 9hr (p<0.05). R‐wave phase shift was delayed compared to BL at 4 and 9hr (p<0.05). Conclusions Acute HX alters the high pass filter properties of intact CA. During HX, CA resembled a low pass filter, where oscillations in ABP were readily transmitted to changes in CBFv across B‐waves (low freq), dampened across M‐waves (middle freq) and delayed across R‐waves (high freq). Supported by NIH HL070362