Human neuropeptide Y (hNPY) potentiates the postjunctional vasoconstrictor effects of ₁-adrenoceptor agonists in animals and in human hand veins in vivo. We therefore hypothesized that such an interaction might also occur in the human arterial bed. With the present single-blind cross-over study in 12 healthy volunteers, the effect of subpressor doses of hNPY on the blood pressure response to ₁-adrenoceptor stimulation was evaluated. Dose-response curves were constructed to intravenously infuse phenylephrine with and without coinfusion with two different doses of hNPY (1.4 and 14.3 pmol · kg¹ · min¹). Blood pressure, heart rate, and forearm blood flow were recorded, and plasma hNPY was determined. During infusion of the higher hNPY dose, which increased hNPY from 24.0 ± 12.0 to 495.1 ± 12.6 pmol/l, blood pressure curves were 2.4-fold shifted toward lower phenylephrine dose rates (P < 0.001). Forearm vascular resistance showed a similar trend, whereas the counterregulatory decrease of heart rate was similar in both groups. In contrast, the lower hNPY dose rate producing a fourfold increase in hNPY concentrations did not modify the response to phenylephrine. This in vivo study in humans demonstrates that hNPY induced potentiating effects on ₁-adrenergic constriction also in the systemic arterial circulation and suggests that circulating hNPY may participate in the control of vascular tone.