Stereoselective pharmacokinetic properties of chloroquine were investigated in humans after a single oral dose of the separate enantiomers. The study was carried out according to a crossover experimental design with a washout period between the administration of each enantiomer. Total blood chloroquine concentrations were measured using an achiral high performance liquid chromatography method. Terminal half-life (t1/2 lambda z) and mean residence time (MRT) were longer for (R)-chloroquine (294h and 388h, respectively) than for (S)-chloroquine (236h and 272h, respectively). The total body clearance was lower for the (R)-enantiomer [136 +/- 38 ml/min (8.16 +/- 2.28 L/h)] than for the (S)-enantiomer [237 +/- 71 ml/min (14.22 +/- 4.26 L/h)]. Although the (R)-stereoisomer remained longer in the body, its volume of distribution (3410 +/- 720L) was lower than than that of (S)-chloroquine (4830 +/- 1490L). Protein binding was different for both chloroquine stereoisomers, with opposite preferential binding to human albumin and alpha 1-acid glycoprotein. Binding to total human plasma amounted to 66.6 +/- 3.3% for (S)-chloroquine and to 42.7 +/- 2.1% for the (R)-enantiomer. De-ethyl-chloroquine concentrations were also different for both enantiomers, resulting in a statistically significant increase in the AUC of (S)-de-ethyl-chloroquine (12.9 +/- 7.4 mg/L.h) compared with (R)-de-ethyl-chloroquine (6.29 +/- 2.18 mg/L.h). With a daily dosage regimen, the divergent pharmacokinetic behaviour of chloroquine enantiomers generates a calculated R:S ratio of blood concentrations amounting to 1:0.7 at steady-state. Insufficient information about stereoselective activity and toxicity of chloroquine stereoisomers prevent further conclusions about the clinical consequences of these pharmacokinetic differences.