Neuropeptide Y (NPY) and ATP are considered cotransmitters with norepinephrine (NE) in sympathetic neurons innervating some blood vessels, including those of the mesentery. A prominent action of NPY is to potentiate the postjunctional contractile effect of NE as well as that of other vasoactive agents. We wished to investigate whether NPY also potentiates the contractile effect of ATP and, if so, to determine which receptor subtype mediates such an effect. The effect of NPY, the NPY-Y1-selective agent Leu31Pro34 NPY, and the NPY-Y2-selective fragment NPY 14-36 on the increase in perfusion pressure produced by ATP was examined in rat perfused mesenteric arterial bed. Results demonstrated that both NPY and Leu31Pro34 NPY but not NPY 14-36 potentiated the increase in perfusion pressure produced by ATP. These results suggest that NPY acts on Y1 receptors to enhance the postjunctional response of ATP. The putative NPY antagonist PYX2, but not the putative antagonists benextramine or PYX1, attenuated the effect of NPY, indicating that PYX2 acts as an NPY antagonist in this system. A major action of NPY is to enhance the postjunctional response of both cotransmitters, ATP and NE at the vascular sympathetic neuroeffector junction in the mesenteric arterial bed, and this may be mediated by NPY-Y1 receptors.