Abstract: Peroxynitrite is implicated in numerous human diseases. Hence, there is considerable interest in potential therapeutic peroxynitrite scavengers. It has been claimed that uric acid is a powerful peroxynitrite scavenger. We previously observed that uric acid is a powerful inhibitor of tyrosine nitration induced by peroxynitrite, but fails to prevent α1-antiproteinase (α1-AP) inactivation induced by peroxynitrite. However, the reactivity of peroxynitrite is significantly modified by bicarbonate and this has not been considered in evaluating the scavenging activity of uric acid and other endogenous antioxidant compounds. In the presence of bicarbonate (25 mM), the ability of uric acid, ascorbate, Trolox, and GSH to inhibit peroxynitrite-mediated tyrosine and guanine nitration is decreased. Protection against peroxynitrite-mediated α1-AP inactivation is also decreased by ascorbate, Trolox, and GSH, but it is enhanced by uric acid. Bicarbonate also inhibits the ability of these compounds to prevent peroxynitrite-mediated ABTS radical cation formation. However, the abilities of these antioxidants to prevent peroxynitrite-mediated bleaching of pyrogallol red are enhanced by bicarbonate. These results show that physiologic concentrations of bicarbonate substantially modify the ability of uric acid to prevent peroxynitrite-mediated reactions. This study highlights the need to use several different assays in the presence of physiologically relevant concentrations of bicarbonate when assessing compounds for peroxynitrite scavenging, in order to avoid misleading results.