A capillary electrophoresis method for the simultaneous quantitation of praziquanteland its main metabolite trans-4-hydroxypraziquantel enantiomers in human plasma was developed and validated using cyclodextrin-modified micellar electrokinetic chromatography. Sample clean-up involved a single-step liquid-liquid extraction of plasma with toluene after the addition of NaCl. The complete enantioselective analysis was obtained in less than 7 min using 2% w/v sulfated beta -cyclodextrin as chiral selector and 20 mmol/L sodium deoxycholate as surfactant, in 20 mmol/L sodium berate buffer, pH 10. A 50 mum x 42 cm uncoated fused-silica capillary was used for the analysis, performed at a voltage of 18 kV and at 20 degreesC. The calibration curves were linear over the 125-625 ng/mL concentration range. The mean recoveries for praziquantel and trans-4-hydroxypraziquantel were up to 96 and 71%, respectively, with good precision. All four enantiomers were quantified at two concentration levels (200 and 600 ng/mL) with precision and accuracy below 15%. The quantitation limit was 50 ng/mL for (-)-(R)-and (+)-(S)-praziquantel and 62.5 ng/mL for (-)-(R)- and (+)-(S)-trans-4-hydroxypraziquantel, using 1 mt of human plasma.