The chiral compound reboxetine is used as a selective noradrenaline reuptake inhibitor (NARI) for the treatment of major depressive disorders. The pharmacokinetic variability of the enantiomers of the drug (S,S- and R,R-reboxetine) was studied using stereoselective high-performance liquid chromatography with mass spectrometric detection in a controlled clinical monotherapy situation (trial I) and a naturalistic clinical setting (trial II). Trial I included patients receiving racemic reboxetine as 6-month monotherapy for treatment of major depressive disorder. Trough level serum samples in steady state were analyzed for the concentration of the reboxetine enantiomers in study weeks 4, 12, and 24. In a therapeutic drug monitoring setting (trial II), 47 patients on doses ranging from 4 to 16 mg daily, including much polypharmacy, trough level steady-state serum samples were analyzed by the same bioanalytical method. Data from trials I and II were assessed to determine the inter- and intraindividual pharmacokinetic outcomes. The results showed that the median S,S/R,R ratio in steady state was 0.5 and ranged from 0.22 to 0.88. It was also shown that women have an approximately 30% higher S,S/R,R ratio than men. The S,S/R,R ratios of reboxetine were not found to correlate with reboxetine concentrations. To investigate the NARI activity of a circulating serum reboxetine concentration, a recalculation of the determined enantiomeric concentrations to previously demonstrated experimental NARI potencies of the drug enantiomers was performed. This partly novel concept of estimating pharmacodynamic activity showed that the serum NARI activity in women tended to be higher than in men at a given reboxetine concentration. In conclusion, the variability in the NARI activity per nmol/L reboxetine and the variability in the concentration outcome of the reboxetine enantiomers may justify the use of enantioselective drug monitoring in the clinic. The gender aspects of the drug have to be further assessed.