Atherosclerosis is associated with endothelial dysfunction and a heightened state of inflammation characterized, in part, by an increase in vascular myeloperoxidase and proteins modified by its principal oxidant, hypochlorous acid (HOCl). Here we examined whether probucol could protect against endothelial dysfunction induced by the two-electron oxidant HOCl. Hypochlorous acid eliminated endothelium-dependent relaxation of rabbit aorta, whereas endothelial function and tissue cGMP was preserved and elevated, respectively, in animals pretreated with probucol. Exogenously added probucol also protected against HOClinduced endothelial dysfunction. In vitro, HOCl oxidized probucol in a two-phase process with rate constants k 1 ؍ 2.7 ؎ 0.3 ؋ 10 2 and k 2 ؍ 0.7 ؎ 0.2 ؋ 10 2 M ؊1 s ؊1 that resulted in a dose-and time-dependent accumulation of probucolderived disulfoxide, 4,4-dithiobis(2,6-di-tert-butyl-phenol) (DTBP), DTBP-derived thiosulfonate, disulfone, and sulfonic acid, together with 3,3,5,5-tetra-tert-butyl-4,4-diphenoquinone (DPQ) as determined by high performance liquid chromatography and mass spectrometry. Like HOCl, selected one-electron oxidants converted probucol into DTBP and DPQ. Also, dietary and in vitro added DTBP protected aortic rings from HOCl-induced endothelial dysfunction and in vitro oxidation by HOCl gave rise to the thiosulfonate, disulfone, and sulfonic acid intermediates and DPQ. However, the product profiles of the in vitro oxidation systems were different from those in aortas of rabbits receiving dietary probucol or DTBP ؎ HOCl treatment. Together, the results show that both probucol and DTBP react with HOCl and protect against HOClinduced endothelial dysfunction, although direct scavenging of HOCl is unlikely to be responsible for the vascular protection by the two compounds.