In patients with obstructive sleep apnea (OSA), substantial elevations of systemic blood pressure (BP) and depressions of oxyhemoglobin saturation (SaO2) accompany apnea termination. The causes of the BP elevations, which contribute significantly to nocturnal hypertension in OSA, have not been defined precisely. To assess the relative contribution of arterial hypoxemia, we observed mean arterial pressure (MAP) changes following obstructive apneas in 11 OSA patients during non-rapid-eye-movement (NREM) sleep and then under three experimental conditions: 1) apnea with O2 supplementation; 2) hypoxemia (SaO2 80%) without apnea; and 3) arousal from sleep with neither hypoxemia nor apnea. We found that apneas recorded during O2 supplementation (SaO2 nadir 93.6% +/- 2.4; mean +/- SD) in six subjects were associated with equivalent postapneic MAP elevations compared with unsupplemented apneas (SaO2 nadir 79-82%): 18.8 +/- 7.1 vs. 21.3 +/- 9.2 mmHg (mean change MAP +/- SD); in the absence of respiratory and sleep disruption in eight subjects, hypoxemia was not associated with the BP elevations observed following apneas: -5.4 +/- 19 vs. 19.1 +/- 7.8 mmHg (P less than 0.01); and in five subjects, auditory arousal alone was associated with MAP elevation similar to that observed following apneas: 24.0 +/- 8.1 vs. 22.0 +/- 6.9 mmHg. We conclude that in NREM sleep postapneic BP elevations are not primarily attributable to arterial hypoxemia. Other factors associated with apnea termination, including arousal from sleep, reinflation of the lungs, and changes of intrathoracic pressure, may be responsible for these elevations.