Abstract: Aim To investig ate therelationship between the clinical actions and the serum concentrations of the enantiomers of (±)-trans tramadol andits active metabolite.Methods 20 postoperative patients weredividedinto two groups and given multiple intraveno us doses of (±)-trans tramadol hydrochloride injection, 400 mg·d^-1 (group A)or 300 mg·d^-1 (group B).The blood samples weretaken at 38 h after the initial dose.The concentrations of the enantiomers of (±)-trans tramadol andits active metabolite, (±)-trans O-demethy ltramadal weredetermined with high performance capillary electrophoresis(HPCE).Results The concentrations of the enantiomers of (±)-trans tramadol, the f requencyand serious level of adversereactions werehigher in group A than in group B.The concentrations of the enantiomers of (±)-trans O-d emethy ltramadal, the analgesic Effect weresimilarbetween group A and group B.Conclusion Thereis much closer relation between the analgesic Effect and the concentration of (+)-O-d emethy ltramadal.The f reque ncyand serious level of adversereactions may be at tributed to the higher concentrations of the enantiomers of (±)-trans tramadol, which arecaused by the saturated metabolism.

Key words: (±)-trans tramadol, (±)-trans O-demethy ltramadol, analg esic effect, adversereaction, high performance capillary electrophoresis