Enantiomers may confer benefits over racemates in therapeutic uses and we developed a chiral separation method of cetirizine enantiomers, a second generation H1 histamine receptor antagonist, in rat plasma. α1-Acidglycoprotein based chiral stationary phase(AGP-CSP), monitored with UV at 230 nm was used to separate the enantiomers. Observed enantioselectivity (α) was 2.0. The AGP-CSP was also used at a preparative scale to isolate the enantiomers with an optical purity of greater than ee 99%. In addition, an analysis was carried out for the cetirizine enantiomers in rat plasma to study the differences of enantiomers in pharmacokinetics. Both (+)- and (-)-cetirizine were separated using a reversed-phase column of AGP, and were detected at the range of 2.5-200 μgml-1 in plasma. Although there was no recognizable differences in pharmacokinetics between the enantiomers in rat, the method appears to be useful for their pharmacokinetic studies. Copyright (C) 2000 Elsevier Science B.V.