Muscle nerve sympathetic activity (MSA), the interval between two R-waves in the ECG, or the interbeat interval (RR-interval), and blood pressure (BP) were recorded in 10 awake patients with obstructive sleep apnea (OSA) and in nine sex- and age-matched controls. Changes in RR-interval and MSA, evoked by sodium nitroprusside-induced reduction of BP, were used to quantitate baroreflex sensitivity. Both the cardiac (expressed as the RR-interval versus mean arterial BP slope) and the muscle sympathetic (mean MSA area versus diastolic BP slope) baroreflex sensitivity were depressed in patients as compared with controls. Cardiac baroreflex slope sensitivity (expressed as a regression coefficient) was 5.5 +/- 1.2 (mean +/- SEM) in patients and 9.6 +/- 0.96 in controls (p < 0.05). The corresponding figures for the sympathetic slope sensitivity were -4.9 +/- 0.9 and -13.1 +/- 2.3, respectively (p < 0.05). Differences remained after stepwise correction for age, body mass index (BMI), and to some extent BP. Resting MSA correlated with cardiac (r = 0.67, p < 0.003) and sympathetic (r = 0.56, p < 0.025) baroreflex sensitivity in the entire study group. We conclude that OSA patients exhibit an impaired baroreflex sensitivity to a hypotensive stimulus, which may represent an adaptive response to changes in BP or hypoxemia occurring in association with nocturnal apneas. Baroreflex adaptation may also contribute to the augmentation of resting MSA observed in OSA patients in this as well as in a previous study.