This study was designed to determine the minimal mean flow velocity and pressure-flow relation necessary to preserve human consciousness. Passive upright tilt provocation was used in conjunction with transcranial Doppler in 80 patients with a history of syncope of unknown etiology. Cerebral blood flow velocity, blood pressure, and heart rate were monitored noninvasively. Forty patients remained asymptomatic, and the rest had clinically induced true syncope or premonitory symptoms. In the asymptomatic group, there was a 23 +/- 16% (p = 0.000) drop in mean flow velocity, but no significant changes in systolic and diastolic blood pressures. In the symptomatic patients, there was a 58 +/- 14% (p = 0.000) drop in mean flow velocity, 37 +/- 23% (p = 0.000) fall in systolic pressure, and 31 +/- 20% (p = 0.000) fall in diastolic pressure. In 80% of symptomatic patients, the critical lower limit of mean flow velocity was at -50% of resting baseline while patients were lying supine. The symptomatic group had lower mean flow velocity and blood pressure responses as compared with the asymptomatic group. The slope and intercept values of the pressure (y axis) to flow velocity (x axis) regression curves indicate a greater degree of impaired autoregulation in the symptomatic group (y = 0.529 x-6.11, r2 = 0.108, p = 0.038) as compared with the asymptomatic (y = 0.317 x + 0.966, r2 = 0.14, p = 0.017). The critical lower limit of cerebral perfusion lies at 50% below baseline supine mean flow velocity.