Abstract: The stereoselective binding of praziquantel (PQT) enantiomers to plasma proteins was studied by equilibrium dialysis. The binding of (±) -PQT and its enantiomers to bovine serum albumin (BSA)and rabbit plasma was unsaturable when initial drug concentrations from 1 to 32μmol/L were used. The binding capacity (nK) of (+)-PQT to BSA (1.47 or 5.88× 10^-4 mol/L)was greater than that of(-)-PQT with a (+) -/(-)-PQT ratio of 1.25 or 1.26, respectively. However, the preferential binding of (-)-PQT to rabbit plasma was found to be 73.7±4.4% for (-)-isomer and 58.3± 10.1% for(+ )-isomer (n=8, P<0.05). Thebinding of(+ )-and (-) -PQT to human plasma was 81.9±4.2% and 83.2±6.9% (n= 10, P>0.05) respectively, which seems to be not stereoselective. These results indicate that there are species differences in the stereoselective binding of PQT enantiomers to plasma proteins.