The interactions of the one-electron oxidants, N-3(.) and Br-2(.-) radicals, with serotonin, tryptamine and their analogues 5-methoxytryptamine, N-methyltryptamine and N(1)-methyltryptophan were studied using the technique of pulse radiolysis with spectrophotometric detection. One-electron oxidation of serotonin results in the formation of an indoloxyl radical with a pK(a) value much less than 3, With tryptamine, N-methyltryptamine and 5-methoxytryptamine their one-electron oxidation gives indolyl radicals with pK(a) values of 4.2, 4.3 and 4.8, respectively. The reactions of OH radicals ((OH)-O-.) with serotonin and tryptamine lead to the formation of the respective (OH)-O-.-adducts, which decay by acid catalyzed water elimination to give almost quantitatively the corresponding indoloxyl and indolyl radicals, respectively. The first-order rate constants determined for water elimination are pH dependent, suggesting that the dehydration reaction is acid and base catalyzed. The (OH)-O-.-adduct of serotonin reacts with oxygen in competition with the dehydration reaction to yield a peroxyl radical adduct, which is tentatively suggested to eliminate HO2.. On the basis of the above findings, the mechanisms for the (OH)-O-.-induced formation of the indoloxyl and indolyl radicals from serotonin and tryptamine respectively are proposed.