We investigated the way in which nasal continuous positive airway pressure (CPAP) affects the circadian profiles of blood pressure (BP) and heart rate (HR) in obstructive sleep apnea syndrome (OSAS) patients. Nine patients with OSAS, confirmed by nocturnal polysomnography, were studied with ambulatory blood pressure monitoring (Colin ABPM-630) during two 48-hour periods, before and during nasal CPAP treatment, at the Stanford University Sleep Disorders Clinic. During each 48-hour monitoring period, blood pressure measurements were taken by the ambulatory device every 30 minutes. During the ambulatory blood pressure recordings, nocturnal sleep time was defined as the period between 0000 hours to 0600 hours and active daytime was defined as the period between 1000 hours and 2000 hours. An average systolic blood pressure > 135 mm Hg during the 48-hour baseline recording was defined as hypertensive. Using these criteria, we selected four hypertensive and five normotensive patients. Average BP (systolic/diastolic) and HR during the 48-hour periods decreased significantly from 148.6/88.2 mm Hg to 138.7/81.4 mm Hg, and from 77.9 beats per minute (bpm) to 67.2 bpm in hypertensives during CPAP treatment (p = 0.04), but there were no significant changes observed in normotensives. Average BP, during the day and at night, decreased from 152.3/91.8 mm Hg to 141.2/85.1 mm Hg and from 133.9/76.8 mm Hg to 125.9/73.7 mm Hg, respectively, in the hypertensives during CPAP, but such changes were not observed in normotensives. Average HR during the day and at night decreased significantly from 85.2 bpm to 72.6 bpm and from 69.8 bpm to 56.5 bpm in the hypertensives (p = 0.04), but not in normotensives.(ABSTRACT TRUNCATED AT 250 WORDS)