Complete ganglion blockade alters dynamic cerebral autoregulation, suggesting links between systemic autonomic traffic and regulation of cerebral blood flow velocity. We tested the hypothesis that acute head-down tilt, a physiologic maneuver that decreases systemic sympathetic activity, would similarly disrupt normal dynamic cerebral autoregulation. We studied 10 healthy young subjects (5 men and 5 women; 21 ± 0.88 yr; 169 ± 3.1 cm; 76 ± 6.1 kg). ECG, beat-by-beat arterial pressure, respiratory rate, end-tidal CO₂, and middle cerebral blood flow velocity were recorded continuously while subjects breathed to a metronome. We recorded data during 5 min periods and averaged responses from 3 Valsalva maneuvers with subjects in both the supine and -10° head-down tilt positions (randomized). Controlled breathing data were analyzed in the frequency domain with power spectral analysis. The magnitude of input-output relations were determined with cross-spectral techniques. Head-down tilt significantly reduced Valsalva phase IV systolic pressure overshoot from 36 ± 4.0 supine to 25 ± 4.0 mmHg head-down (P = 0.03). Systolic arterial pressure spectral power at the low frequency decreased from 5.7 ± 1.6 supine to 4.4 ± 1.6 mmHg² head-down (P = 0.02), and mean arterial pressure spectral power at the low frequency decreased from 3.3 ± 0.79 supine to 2.0 ± 0.38 mmHg² head-down (P = 0.05). Head-down tilt did not affect cerebral blood flow velocity or the transfer function magnitude and phase angle between arterial pressure and cerebral blood flow velocity. Our results show that in healthy humans, mild physiological manipulation of autonomic activity with acute head-down tilt has no effect on the ability of the cerebral vasculature to regulate flow velocity.