Abstract tert-Butyl hydroperoxide ( t-BOOH), an inducer of oxidative stress in vitro, elicits constrictor responses of the isolated, rat kidney and mesenteric arteries perfused (5 ml/min) with physiological salt solutions (PSS) at 37°C gassed with carbogen. We hypothesized that generation of superoxide anions (O 2 −) accounts for these responses. We assessed responses to t-BOOH in preparations with/without endothelium, and in the absence/presence of antioxidant compounds, catalase and tempol, scavengers of hydroxyl (OH −) radical and O 2 −, respectively. t-BOOH (0.01–50 μmol) induced (expressed as % of 50 μmol KCl vasoconstriction) were abolished by endothelium denudation, perfusion with Ca 2+-free PSS and by nifedipine, (1 nM). Infusion of t-BOOH (0.1 μM) did not significantly (P > 0.05) affect phenylepherine E max in the mesenteric arteries, however it reduced phenylepherine E max responses in the kidney from 94.9 ± 3.9 % to 64.7 ± 4.7 %. Nitroblue tetrazolium, as well as α-phenyl N- tert-butyl nitrone, at 100 μM, but not catalase (500 IU) significantly attenuated t-BOOH (10 μmol) vasoconstrictor responses. Tempol (100 μM), a membrane permeable antioxidant, also significantly reduced t-BOOH (10 μmol) responses from 17.0 ± 1.9 % (control) to 9.6 ± 0.5 % (+tempol) in the mesenteric arteries and from 40.4 ± 4.2 % (control) to 20.7 ± 1.5 % (+tempol) in the kidney. Our data suggest that t-BOOH elicits vasoconstriction via two distinct mechanisms: (i) increased influx of extracellular Ca 2+, and (ii) generation of free radicals including O 2 − anions.