Previously we have demonstrated functional nitric oxide synthase (NOS) 1 in large arteries. Because resistance arteries largely determine blood pressure, this study examined whether functional NOS 1 also exists in resistance arteries. Phenylephrine (PE) contraction was measured in the absence and presence of the NOS 1 inhibitor N5-(1-imino-3-butenyl)-L-ornithine (VNIO) in isolated mesenteric resistance arteries (endothelium intact and denuded) from Sprague-Dawley rats. For NOS 1 activity and expression, the mesenteric arterial bed was separated into cytosolic and particulate fractions. NOS activity was assayed by measuring the conversion of [3H]arginine to [3H]citrulline inhibited by a nonselective NOS inhibitor or VNIO. VNIO increased PE sensitivity in endothelium-intact and -denuded arteries. In cytosolic and particulate fractions of the arterial bed, ∼40% of NOS activity was inhibited by VNIO. Immunoprecipitation and Western blot analysis revealed two NOS 1 immunoreactive bands. One band corresponded to the rat brain isoform, whereas the second was of a slightly lower molecular mass. The cytosolic fraction contained both isoforms; however, the particulate fraction had only the lower molecular mass form. These studies demonstrate the existence of functional NOS 1 in resistance arteries.