To assess the acute effects of nasal continuous positive airway pressure (CPAP) on the 24-hour blood pressure and the secretion of catecholamines in urine and plasma, we investigated the changes in the 24-hour blood pressure and urinary and plasma concentrations of epinephrine (E) and norepinephrine (NE) in 26 men with obstructive sleep apnea (OSA) with and without nasal CPAP. Nasal CPAP resulted in significant decreases in the daytime diastolic pressure (from 86 +/-16 mmHg to 83+/-12 mmHg), the nighttime diastolic pressure (from 81+/-12 mmHg to 77+/-9 mmHg) and the nighttime systolic pressures (from 125+/-15 mmHg to 120+/-10 mmHg). There was no significant difference between patients with and without CPAP in the daytime or nighttime urinary E level, but patients who received CPAP showed a significant decrease in daytime urinary NE level (from 156+/-112 microg/14h to 119+/-101 microg/14h) and nighttime urinary NE level (from 143+/-91 microg/10h to 112+/-65 microg/10h). The morning plasma level of NE also decreased (from 371+/-181 pg/ml to 273 +/-148 pg/ml) in patients who received nasal CPAP (p<0.02), but the plasma level of E remained unchanged. There were no correlations between PSG parameters and the reductions in blood pressure and the catecholamine levels induced by nasal CPAP. These findings suggest that OSA contributes, at least in part, to the development of systemic hypertension by increasing sympathetic nervous activity.