The present study examined the possibility that blood vessels of the rat and cow contain arginine vasopressin (AVP) and that the vascular stores of this potent vasoconstrictor are of local, rather than blood-borne, origin. Vessels from rat or cow were homogenized in acid, and the supernatants were assayed for AVP by radioimmunoassay. AVP immunoreactivity was detected in all vessels taken from the rat including aorta, mesenteric artery, renal artery, vena cava, and renal vein, as well as atria of the heart and also in cow aorta. Blood vessels from hypophysectomized and Brattleboro rats also contained AVP at levels similar to those of intact control rats. Further characterization of this immunoreactive material showed that, in both radioimmunoassay and radioreceptor assay, vascular extracts produced competition curves, which were parallel to those of synthetic AVP. Additionally, immunoreactive AVP in aortic extracts comigrated with synthetic AVP and pituitary extract on both high-pressure liquid chromatography and Sephadex G-25 chromatography. Furthermore, intravenous administration of the aortic extract produced pressor responses in conscious rats, and these responses were significantly attenuated by pretreatment with a specific AVP V1-receptor antagonist. These data demonstrate that blood vessels contain an AVP-like peptide that appears indistinguishable from authentic AVP and further suggest that this vascular peptide is of local origin.