Cerebral circulation was studied in six patients with obstructive sleep apnea (OSA) by transcranial Doppler. Cerebral blood flow velocity (CBFV), pulsatility index (Pi), intra-arterial radial blood pressure, transcutaneous blood gases, and respiration were recorded during sleep with or without apneas. There was a concomitant increase of mean arterial pressure (Pa) and CBFV by 11 +/- 6% (p < 0.001) and 15 +/- 6% (p < 0.001) compared with baseline at 5.1 +/- 2.4 and 5.3 +/- 2.6 s, respectively, after apnea termination. Pa and CBFV decreased to a minimum of -8 +/- 2% (p < 0.001) and -23 +/- 8% (p < 0.001), respectively, below baseline at 19.8 +/- 5.0 and 19.4 +/- 4.5 s after apnea. Values returned to baseline within 60 s, except during repetitive apneas, which were associated with prolonged periods of reduced Pa and CBFV. Pi changed inversely to CBFV and rose to 34 +/- 15% (p < 0.001) above baseline 19.5 +/- 5.5 s after the end of apnea because of a decrease in diastolic CBFV. There was a close correlation between Pa and CBFV (r = 0.67, p < 0.001), indicating that cerebral autoregulation is insufficient to protect the brain from rapid systemic pressure changes in OSA. The findings suggest that apneas during sleep are associated with profound changes in cerebral blood flow. Apnea-induced hypoxemia combined with reduced cerebral perfusion may predispose to nocturnal cerebral ischemia in patients with OSA.