On reperfusion of ischemic tissue, a prolonged phase of vasoconstriction occurs, the mechanism of which is poorly understood. However, it is known that peroxynitrite (ONOO-) is formed during reperfusion. In this study the contractile properties of ONOO- were investigated in Wistar rat middle cerebral arteries. The effects of ONOO- on vessel diameter were dose dependent. Low-dose ONOO- (10 microM) caused vessels to constrict by 15%. At an intermediate concentration of 25 microM, the effect of ONOO- was variable, whereas at the highest concentration (100 microM), vessels underwent persistent dilation and became insensitive to the endogenous vasoconstrictor 5-hydroxytryptamine. At the single cell level, ONOO- caused cerebral artery smooth muscle cells to contract. Reduced, but not oxidized, glutathione completely inhibited the contractile action of ONOO- on single cells. Vehicle and decomposed ONOO- each had minimal effect on cell length. These data show that ONOO- is a contractile agonist of middle cerebral arteries, at the single cell and whole vessel levels, suggesting that formation of ONOO- may contribute mechanistically to ischemic brain injury during stroke. Moreover, relatively high concentrations of ONOO- result in vascular paralysis.