We tested the hypothesis that the ability of coronary arteries to withstand functional damage from superoxide is altered by overnight exposure of the arteries to a physiological concentration of ß-estradiol. Female porcine coronary arterial rings were incubated in an O₂/CO₂ incubator, under normoxic conditions, at 37°C for 22-24 hours. Arteries were then placed in baths containing a physiological salt solution at 37°C with 95% O₂/ 5%CO₂ for isometric force recordings. In rings from 14 female pigs, vasorelaxation to A23187 and DEA-NONOate was determined with and without prior 15' exposure to 400 µM pyrogallol. Sensitivity (-logM ED₅₀) and maximum relaxation to A23187, but not DEA- NONOate, were significantly impaired by exposure to pyrogallol (Pyrogallol treated: 7.39±0.09, 82±5%; Control: 7.76±0.11, 99±1% (SE), p<0.01 and <0.05, respectively). This effect was attenuated by concurrent exposure to equimolar ascorbate. Arterial rings from 12 separate female pigs were incubated for 22-24 hours with or without 1M ß-estradiol prior to pyrogallol exposure. ß-estradiol significantly enhanced arterial sensitivity to A23187 and prevented pyrogallol impairment without affecting DEA-NONOate responses. Therefore, superoxide meditated endothelial damage and impaired endothelium dependent relaxation of coronary arteries is prevented by overnight exposure of the arteries to a physiological concentration of ß-estradiol.