Abstract: AimTo investigate the stereoselectivity in biliary excretion of trans tramadol (trans T) and trans O-demethyltramadol (M1) in rats. MethodsAfter a single intravenous dose of trans T hydrochloride (10 mg·kg^-1) or M1 (2.5 mg·kg^-1) to rats, the bile was collected for 30 min, then, blood was obtained from the heart. The enantiomers of trans T, M1 and M1 conjugated with glucuronic acid (M1_c) in the bile and plasma were analyzed by high performance capillary electrophoresis (HPCE). ResultsAfter the rats were given trans T, the bile concentrations of (+)-trans T were higher than those of (-)-trans T, and the (+)/(-)-trans T ratios were lower compared with those in the plasma. After the rats were given M1, the bile concentrations of (+)-M1 were higher than those of (-)-M1, and the bile concentrations of (+)-M1_c were lower than those of (-)-M1_c. The glucuronidation rate of (+)-M1 was lower than that of (-)-M1 in the bile. ConclusionThe biliary excretion of trans T and M1 was stereoselective, (+)-trans T and (-)-M1 being preferentially excreted.