Detailed kinetic studies have been performed for reactions of aroxyl (ArO center dot) and alpha-tocopheroxyl (alpha-Toc(center dot)) radicals with five catecholamines (CAs) (dopamine (DA), norepinephrine (NE), epinephrine (EN), and 5- and 6-hydroxydopamine (5- and 6-OHDA)) and two catechins (epicatechin (EC) and epigallocatechin gallate (EGCG)) to clarify the free-radical-scavenging activity of CAs. Second order rate constants (k(s) and k(r)) for reactions of ArO center dot and alpha-Toc(center dot) radicals with the above antioxidants were measured in 2-propanol/water (5:1, v/v) solution at 25.0 degrees C, using single and double-mixing stopped-flow spectrophotometries, respectively. Both the rate constants (k(s) and k(r)) increased in the order NE < EN < DA < EC < 5-OHDA < EGCG < 6-OHDA. The k(s) and k(r) values of 6-OHDA are large and comparable to the corresponding values of ubiquinol-10 and sodium ascorbate, which show high free-radical-scavenging activity. The ultraviolet visible absorption of alpha-Toc(center dot) (lambda(max) = 428 nm), which was produced by the reaction of alpha-tocopherol (alpha-TocH) with ArO center dot, disappeared under the coexistence of CAs due to the alpha-TocH-regeneration reaction. The results suggest that the CAs may contribute to the protection from oxidative damage in nervous systems, by scavenging free radicals (such as lipid peroxyl radical) and regenerating alpha-TocH from the alpha-Toc(center dot) radical.