This study examines whether longitudinal antioxidant treatment initiated in prehypertensive spontaneously hypertensive rats (SHR) can attenuate vascular oxidant stress and prevent blood pressure elevation during development. Male SHR and age-matched Wistar-Kyoto rats (WKY) were treated from 6 to 11 weeks of age with Tempol (4-hydroxy-2,2,6,6-tetramethylpiperidinoxyl) (1 mmol/l in drinking water), a membrane-permeable superoxide dismutase mimetic. Mean systolic blood pressures (SBPs) were measured by tail-cuff Agonist-induced and basal O2- production was measured in thoracic aortas of 6- and 11-week-old SHR and WKY by lucigenin-derived chemiluminescence and oxidative fluorescent microscopy, respectively. SBP of 6-week-old SHR (131 +/- 5 mmHg) and WKY (130 +/- 4 mmHg) were not different; however, 11-week-old SHR SBP (171 +/- 4 mmHg) was significantly greater (p = .0001) than 11-week-old WKY SBP (143 +/- 5 mmHg). Tempol treatment completely, but reversibly, prevented this age-related rise in SHR SBP (SHR + Tempol: 137 +/- 4 mmHg; p < .0001 versus untreated SHR). Agonist-induced vascular O2- was increased in 6- (p = .03) and 11-week-old SHR (p < .0001) and 11-week-old WKY (p = .03) but not in 6-week-old WKY. Long-term Tempol treatment significantly lowered O2- production in both strains. Basal O2- measurements in both 6- and 11-week-old SHR were qualitatively increased compared with age-matched WKY; this increase in SHR was inhibited with in vitro Tempol treatment. These data show that antioxidant treatment to reduce oxidative stress prevents the age-related development of high blood pressure in an animal model of genetic hypertension.