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The present study was undertaken to determine the vascular actions of peroxynitrite (ONOO−), the product of superoxide and nitric oxide (NO), in isolated canine cerebral arteries and to gain insight into its potential mechanisms of action. In the absence of any vasoactive agent, ONOO− (from 10−7 to 10−6 M) was able to reduce the basal tension. In prostaglandin F2α-precontracted canine basilar arterial rings, ONOO− elicited concentration-dependent relaxation at concentrations from 10−8 to 10−5 M. The effective concentrations producing approximately 50% maximal relaxation (EC50) to ONOO− were 4.06 × 10−6 and 4.12 × 10−6 M in intact and denuded rings, respectively (P > 0.05). No significant differences in relaxation responses were found in ring preparations with or without endothelium (P > 0.05). The presence of either 5 μM methylene blue (MB) or 5 μM 1H-[1,2,4]oxadiazolo-[4,3-α]quinoxalin-1-one (ODQ) significantly inhibited the relaxations induced by ONOO−. Tetraethylammonium chloride (T-2265) significantly decreased the ONOO−-induced relaxations in a concentration-dependent manner. However, ONOO− had no effect on rings precontracted by high KCL (P > 0.05). Addition of low concentrations of calyculin A (50 nM) was able to abolish the ONOO−-induced relaxation. Furthermore, ONOO− significantly inhibited calcium-induced contractions of K+-depolarized canine cerebral rings in a concentration-related manner. Lastly, a variety of pharmacological agents and antagonists including L-NMMA, l-arginine, indomethacin, atropine, naloxone, diphenhydramine, cimetine, glibenclamide, haloperidol, etc., did not influence the relaxant effects of ONOO− on the rings. Our new results suggest that ONOO−-triggered relaxation, on canine cerebral arteries, is mediated by elevation of cyclic guanosine monophosphate (cGMP) levels, membrane hyperpolarization via K+ channel activation, activation of myosin light chain phosphatase activity, and interference with calcium movement and cellular membrane Ca2+ entry.