Following p.o. administration of 14C-labelled rac.-1-(e)-(m-methoxyphenyl)-2-(e)-dimethylaminomethyl-cyclohexan-1-(a)-ol hydrochloride (tramadol hydrochloride, CG 315, Tramal) to mice, hamsters, rats, guinea pigs, rabbits, dogs and man the metabolic pathways were investigated and the results compared. After synthesis of the reference substances the metabolites were identified by co-chromatography using both TLC (thin-layer chromatography) and HPLC (high-performance liquid chromatography) methods, by co-crystallization and by gas chromatography-mass spectrometry. In all species the main metabolic pathways are N- and O-demethylation (phase I reactions) and conjugation of O-demethylated compounds (phase II reactions). 11 metabolites are known, 5 arising by phase I reactions (M1 to M5) and 6 by phase II reactions (glucuronides and sulfates of M1, M4 and M5). The 5 phase I metabolites are mono-O-demethyl-tramadol (M1), mono-N-demethyl-tramadol (M2), di-N-demethyl-tramadol (M3), tri-N,O-demethyl-tramadol (M4) and di-N,O-demethyl-tramadol (M5). The biotransformation scheme of tramadol is qualitatively identical in man, dog, rabbit, guinea pig, rat, hamster and mouse. In all species M1 and M1-conjugates, M5 and M5-conjugates and M2 are the main metabolites, whereas M3, M4 and M4-conjugates were only formed in minor quantities. Following p.o. administration to man and animals 14C-tramadol are rapidly and almost completely absorbed. The unchanged drug and metabolites are mainly excreted via kidneys. The cumulative renal excretion of total radioactivity accounts for approximately 90% in man and varies from 86 to 100% in mouse, hamster, rat, guinea pig, rabbit and dog; the residual of the applied radioactivity appears in the feces. Apparently tramadol is metabolized much more rapidly in animals than in man. For that reason there are appreciable differences between man and animals in the amount of tramadol excreted unchanged in the urine (about 30% and 1% of the p.o. dose, respectively). After incubation with beta-glucuronidase and arylsulfatase at least 81% of the excreted radioactivity could be extracted from the urine of man animals (with the exception of the guinea pig and the rabbit). In man all extractable metabolites were identified.