In order to study blood pressure, adrenergic, and cholinergic activity in severe obstructive sleep apnea (OSA), 14 patients with apnea index (AI) greater than 30 apneas/h, mean apnea time index greater than 20 sec were studied before and with nasal Continuous Positive Airway Pressure (nCPAP). Sleep, respiration, arterial blood pressure, arterial plasma catecholamines [norepinephrine (NE) and epinephrine (E)], and plasma pancreatic polypeptide (PP) were measured without and after seven days treatment with nCPAP. Initially in the apnea, arterial blood pressure decreases, and thereafter steadily increases and shows a maximum during the first breath terminating the apnea. With nCPAP the arterial pressure variations disappeared. Awake morning systolic (SBP) and diastolic (DBP) blood pressure decreased the nCPAP treatment. A significant relation between the reduction in AI and the reduction in morning awake SBP and DBP with nCPAP treatment was observed. No significant changes in NE were observed with nCPAP treatment. Epinephrine decreased and PP increased significantly with nasal nCPAP treatment. The systolic and diastolic blood pressure reduction was significantly related to the decrease in E and the increase in PP. No association between sleep stages and plasma NE, E, and plasma-PP was found before treatment. With nCPAP treatment plasma-PP was higher during non-rapid eye movement (NREM) stage 2 to 4 sleep than during rapid eye movement (REM) sleep. Thus, morning awake arterial blood pressure and nocturnal arterial blood pressure decrease with nCPAP treatment in sleep apnea patients. These hemodynamic changes are related to the decrease in AI and sympathetic activity and the increase in parasympathetic activity.