1. The biotransformation of [1-14C]dodecyldimethylamine oxide (DDAO) was studied in rats, rabbits and humans. 2. Metabolites eliminated in 0-24 h urine after oral dosing were separated by cation exchange chromatography into at least ten separate peaks for each species. Several prominent urinary metabolites were identified using reverse isotope dilution, g.l.c.-mass spectrometry and n.m.r. 3. Metabolite structures suggested metabolic pathways that included omega, beta-oxidation of the aliphatic chain, amine oxide reduction, and aliphatic mid-chain hydroxylation. 4. Comparison was made of the 0-24 h urinary metabolites from males of each species. N,N-Dimethyl-4-aminobutyric acid and its N-oxide accounted for 28% of the dose in humans, 28% in rabbits and 23% in rats. These species excreted 44, 51 and 60% of the dose, respectively, in 24 h. Therefore, humans and rabbits were more efficient than rats in degrading the alkyl chain to compounds containing four-carbon acids. In contrast to rabbits and humans, rats excreted greater than 12% of the dose as long-chain compounds. At least half of these compounds were C-hydroxylated amino-alcohols. 5. Under the conditions employed, the rabbit more closely approximates the metabolism of DDAO by human than does the rat.