The cation radical of each of the phenothiazine tranquilizers is a likely intermediate in the metabolism of the drugs to at least two of the three major metabolic classes, the sulfoxides and the hydroxylated derivatives. Previous work has shown that the reactions of the radical are highly dependent on the environment, particularly the presence of nucleophiles. The present report discusses the effect of cation radical structure on the formation of sulfoxide and hydroxylated metabolites in vitro. Cyclic voltammetry, spectrophotometry, and liquid chromatography were used to examine reactions of various phenothiazine radicals in aqueous buffers. A radical with a three-carbon aliphatic side chain (e.g., chlorpromazine) forms solely sulfoxide and parent unless amine nucleophiles are present, in which case hydroxylation occurs. A shorter side chain (e.g., promethazine) causes radical dimerization and pronounced hydroxylation, regardless of external nucleophiles. A piperazine side chain (e.g., fluphenazine) promotes hydroxylation, with some sulfoxide observed. The results indicate that a deprotonated amine is necessary for hydroxylation and that the amine may be present in the original drug rather than an external nucleophile. In addition to information about cation radical reactions, the redox properties of several different phenothiazines are presented.