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Recent studies suggest that activation of the sympathetic nervous system either directly or indirectly influences cerebrovascular tone in humans even within the autoregulatory range. In 6 healthy subjects (aged 29+/-4 years), we used transcranial Doppler sonography to determine cerebral blood flow velocity during sympathetic activation elicited through head-up tilt (HUT) and sympathetic deactivation through ganglionic blockade. PaCO(2) was manipulated through hyperventilation and CO(2) breathing (5%). With subjects in the supine position and during HUT, mean arterial pressure was not influenced by PaCO(2). During ganglionic blockade, mean arterial pressure decreased markedly with hyperventilation (-13+/-1.9 mm Hg). Manipulation of sympathetic tone elicited only mild changes in cerebral blood flow (64+/-5.8 cm/s supine, 58+/-4.9 cm/s upright, and 66+/-6.2 cm/s during ganglionic blockade; P:=0.07 by ANOVA). The slope of the regression between PaCO(2) and mean velocity was 1.6+/-0.18 cm/(s. mm Hg) supine, 1.3+/-0.14 cm/(s. mm Hg) during HUT, and 2.3+/-0.36 cm/(s. mm Hg) during ganglionic blockade (P:<0.05). Spontaneous PaCO(2) and ventilatory response to hypercapnia were also modulated by the level of sympathetic activity. Changes in sympathetic tone have a limited effect on cerebral blood flow at normal PaCO(2) levels. However, the sympathetic nervous system seems to attenuate the CO(2)-induced increase in cerebral blood flow. This phenomenon may indicate a moderate direct effect of the sympathetic nervous system on the cerebral vasculature. Furthermore, sympathetic activation tends to increase ventilation and thus can indirectly increase cerebrovascular tone.