OUR understanding of the regulation of vascular tone has been extended since the identification of vasoactive agents such as the atrial natriuretic peptides¹, endothelial-derived relaxing factor² and endothelin³. Unidentified vasopressive agents have been found in platelets⁴. Here we isolate these vasopressors and identify them as diadenosine pentaphosphate (AP₅A) and diadenosine hex-aphosphate (AP₆A) by chromatography, mass spectrometry, ultraviolet spectroscopy and enzymatic cleavage. In the vasculature of isolated perfused rat kidney, both diadenosine phosphates were active at a concentration of 10⁻⁹ M; in aortic rings, contractions were elicited at 10⁻⁸ M.Intra-aortic injection in the rat caused a prolonged increase in blood pressure. We conclude that AP₅A and AP₆A may play a part in local vasoregulation and possibly in the regulation of blood pressure.