The role of neuropeptide Y (NPY) in the neurotransmission of calcitonin gene-related peptide (CGRP)-containing vasodilator nerves was investigated in perfused mesenteric vascular beds of the rat with active tone. NPY (5 and 10 nM) significantly inhibited the neurogenic vasodilation induced by perivascular nerve stimulation (1-8 Hz). However, NPY had no effect on vasodilation in response to bolus infusion of rat CGRP (10 and 100 pmol). These results suggest that NPY modulates the release of CGRP from CGRP-containing nerves in mesenteric resistance vessels.