Although the cerebrovasculature is known to be exquisitely sensitive to CO2, there is no consensus on whether the sympathetic nervous system plays a role in regulating cerebrovascular responses to changes in arterial CO2. To address this question, we investigated human cerebrovascular CO2 reactivity in healthy participants randomly assigned to the α1-adrenoreceptor blockade group (9 participants; oral prazosin, 0.05 mg/kg) or the placebo control (9 participants) group. We recorded mean arterial blood pressure (MAP), heart rate (HR), mean middle cerebral artery flow velocity (MCAV mean), and partial pressure of end-tidal CO2 (PETCO2) during 5% CO2 inhalation and voluntary hyperventilation. CO2 reactivity was quantified as the slope of the linear relationship between breath-to-breath PETCO2 and the average MCAvmean within successive breathes after accounting for MAP as a covariate. Prazosin did not alter resting HR, PETCO2, MAP, or MCAV mean. The reduction in hypocapnic CO2 reactivity following prazosin (-0.48 ± 0.093 cm·s -1·mmHg-1) was greater compared with placebo (-0.19 ± 0.087 cm·s-1·mmHg-1; P < 0.05 for interaction). In contrast, the change in hypercapnic CO2 reactivity following prazosin (-0.23 cm·s-1·mmHg -1) was similar to placebo (-0.31 cm·s -1·mmHg-1; P = 0.50 for interaction). These data indicate that the sympathetic nervous system contributes to CO2 reactivity via α1-adrenoreceptors; blocking this pathway with prazosin reduces CO2 reactivity to hypocapnia but not hypercapnia.