Measurements of aroxyl radical (ArO center dot)-scavenging rate constants (k(s)(AOH)) of antioxidants (AOHs) (alpha-tocopherol (alpha-TocH), ubiquinol-10 (UQ(10)H(2)), and sodium ascorbate (Na+AsH-)) were performed in 2-propanol/water (2-PrOH/H2O, 5/1, v/v) solution using stopped-flow spectrophotometry. k(s)(AOH) values were measured not only for each AOH but also for the mixtures of two AOHs ((i) alpha-TocH and UQ(10)H(2) and (ii) alpha-TocH and Na+AsH-). A notable synergistic effect that the k(s)(AOH) values increase 1.6, 2.5, and 6.8 times for alpha-TocH, UQ(10)H(2), and Na+AsH-, respectively, was observed for the solutions including two kinds of AOHs. Furthermore, measurements of the regeneration rates of alpha-tocopheroxyl radical (alpha-Toc(center dot)) to alpha-TocH by UQ(10)H(2) and Na+AsH- were performed in 2-PrOH/H2O using double-mixing stopped-flow spectrophotometry. Second-order rate constants (k(r)) obtained for UQ(10)H(2) and Na+AsH- were 2.01 x 10(5) and 1.19 x 10(6) M-1 s(-1), respectively. In fact, UV-vis absorption of alpha-Toc(center dot) (lambda(max) = 428 nm), which had been produced by reaction of alpha-TocH with ArO center dot, disappeared under the existence of UQ(10)H(2) or Na+AsH- due to the above fast regeneration reaction. The result indicates that the prooxidant effect of alpha-Toc(center dot) is suppressed by the coexistence of UQ(10)H(2) or Na+AsH-. As alpha-TocH, UQ(10)H(2), and ascorbate monoanion (AsH-) coexist in relatively high concentrations in plasma, blood, and various tissues, the above synergistic effect, that is, the increase of the free-radical-scavenging rate and suppression of the prooxidant reaction, may function in biological systems.