Our understanding of the regulation of vascular tone has been extended since the identification of vasoactive agents such as the atrial natriuretic peptides, endothelial-derived relaxing factor and endothelin. Unidentified vasopressive agents have been found in platelets. Here we isolate these vasopressors and identify them as diadenosine pentaphosphate (AP5A) and diadenosine hexaphosphate (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.