The arterial pressure elevations that accompany sleep apneas may be caused by chemoreflex stimulation, negative intrathoracic pressure, and/or arousal. To assess the neurocirculatory effects of arousal alone, we applied graded auditory stimuli during non-rapid-eye-movement (NREM) sleep in eight healthy humans. We measured muscle sympathetic nerve activity (intraneural microelectrodes), electroencephalogram (EEG; C4/A1 and O1/A2), arterial pressure (photoelectric plethysmography), heart rate (electrocardiogram), and stroke volume (impedance cardiography). Auditory stimuli caused abrupt increases in systolic and diastolic pressures (21 +/- 2 and 15 +/- 1 mmHg) and heart rate (11 +/- 2 beats/min). Cardiac output decreased (-10%). Stimuli that produced EEG evidence of arousal evoked one to two large bursts of sympathetic activity (316 +/- 46% of baseline amplitude). Stimuli that did not alter EEG frequency produced smaller but consistent pressor responses even though no sympathetic activation was observed. We conclude that arousal from NREM sleep evokes a pressor response caused by increased peripheral vascular resistance. Increased sympathetic outflow to skeletal muscle may contribute to, but is not required for, this vasoconstriction. The neurocirculatory effects of arousal may augment those caused by asphyxia during episodes of sleep-disordered breathing.