The mechanism of vasovagal syncope during orthostasis in humans is unknown. Opioid receptors have been implicated in the vasovagal-like responses to hemorrhagic hypotension in conscious animals. We sought to determine if opioid receptor blockade with naloxone (mu receptor antagonist) would prevent or delay the onset of vasovagal syncope in humans. Three protocols were performed in which heart rate, arterial pressure, sympathetic nerve activity, thoracic impedance and forearm vascular resistance were measured during stepwise steady-state increments of lower body negative pressure (LBNP) in nine healthy volunteers. In protocol 1, duplicate trials of LBNP to syncope or -60 mmHg were performed with a 30-45 minute rest period separating the trials. No significant differences in any physiologic responses or cumulative stress tolerance were found. In protocol 2, graded LBNP was repeated after administration of saline or naloxone (0.1 mg/kg) in six subjects in which vasovagal syncope occurred prior to -60 mmHg LBNP. The peak increase of sympathetic nerve activity during LBNP was augmented after naloxone (P = 0.02), but the occurrence of vasovagal syncope was not prevented nor was the cumulative stress tolerated affected (P = 0.42). The heart rate and arterial pressure responses to LBNP were not affected by naloxone. Similarly, in protocol 3, naloxone given just prior to the onset of pre-syncopal symptoms did not alter the physiologic response or the occurrence of vasovagal syncope. These data show that naloxone does not prevent or delay the onset of vasovagal syncope in humans which suggests that mu opioid receptors do not mediate the vasovagal response.