Long duration habitation on the International Space Station (ISS) is associated with chronic elevations in arterial blood pressure in the brain compared with normal upright posture on Earth and elevated inspired CO₂. Although results from short-duration spaceflights suggested possibly improved cerebrovascular autoregulation, animal models provided evidence of structural and functional changes in cerebral vessels that might negatively impact autoregulation with longer periods in microgravity. Seven astronauts (1 woman) spent 147 ± 49 days on ISS. Preflight testing (30–60 days before launch) was compared with postflight testing on landing day ( n = 4) or the morning 1 ( n = 2) or 2 days ( n = 1) after return to Earth. Arterial blood pressure at the level of the middle cerebral artery (BP_MCA) and expired CO₂ were monitored along with transcranial Doppler ultrasound assessment of middle cerebral artery (MCA) blood flow velocity (CBFV). Cerebrovascular resistance index was calculated as (CVRi = BP_MCA/CBFV). Cerebrovascular autoregulation and CO₂ reactivity were assessed in a supine position from an autoregressive moving average (ARMA) model of data obtained during a test where two breaths of 10% CO₂ were given four times during a 5-min period. CBFV and Doppler pulsatility index were reduced during −20 mmHg lower body negative pressure, with no differences pre- to postflight. The postflight indicator of dynamic autoregulation from the ARMA model revealed reduced gain for the CVRi response to BP_MCA ( P = 0.017). The postflight responses to CO₂ were reduced for CBFV ( P = 0.056) and CVRi ( P = 0.047). These results indicate that long duration missions on the ISS impaired dynamic cerebrovascular autoregulation and reduced cerebrovascular CO₂ reactivity.