The role of the arterial baroreflex in the cardiovascular changes associated with the obstructive sleep apnea syndrome (OSAS), and the effect of nasal continuous positive airway pressure (CPAP) treatment on baroreflex function during sleep are unknown. Baroreflex control of heart rate was studied in 29 normotensive patients with OSAS under no treatment, in 11 age-matched control subjects, and in 10 patients at CPAP withdrawal after 5.5 +/- 3.7 (range 3-14) months of treatment. Baroreflex control of heart rate was assessed by "sequence method" analysis of continuous blood pressure recordings (Finapres) obtained during nocturnal polysomnography. In untreated OSAS, baroreflex sensitivity (BRS) was low during wakefulness and non-rapid eye movement (REM) stage 2 sleep compared with control subjects, and correlated inversely with mean lowest Sa(O(2)) and the blood pressure increase after apneas. After CPAP treatment, the apnea-hypopnea index was lower, and mean lowest Sa(O(2)) higher than before treatment. After CPAP, patients were more bradycardic, blood pressure and its standard deviation decreased as Sa(O(2)) improved in non-REM stage 2 sleep, and BRS increased (nocturnal wakefulness: +59%; non-REM stage 2 sleep: +68% over pretreatment values). Our data suggest that baroreflex dysfunction in OSAS may be at least partly accounted for by nocturnal intermittent hypoxemia, and can be reversed by long-term CPAP treatment.