1. Stereoselectivity in the disposition of hydroxychloroquine was investigated in 23 healthy males following a single oral dose of 200 mg racemic HCQ (rac-HCQ) sulphate. Total concentrations (R+S) and R/S ratios of HCQ and its metabolites were measured by stereoselective h.p.l.c. 2. HCQ was detected in whole blood and urine, up to 91 and 85 days after dosing, respectively. Metabolites could not be detected in whole blood while in urine detectable concentrations were still present after 85 days. The blood concentrations of HCQ enantiomers were measurable until 168 h post-dose. 3. R(-)-HCQ accounted for 62 +/- 3% (mean +/- s.d.) of the AUC of rac-HCQ AUC. The elimination half-life of S(+)-HCQ (457 +/- 122 h) was significantly shorter than that of R(-)-HCQ (526 +/- 140 h), partly due to its faster urinary excretion and hepatic metabolism. Its renal clearance was twice that of R(-)-HCQ (4.61 +/- 4.01 vs 1.79 +/- 1.30 1 h-1), and metabolites derived from the S-isomer represented 80-90% of the urinary recovery of the dose. 4. Over 85 days, 4.4 +/- 2.9 and 3.3 +/- 1.8% of the dose was recovered in urine as unchanged S(+)-HCQ and R(-)-HCQ, respectively. For the first 2 weeks, S(+)-HCQ excretion rate clearly surpassed that of R(-)-HCQ whereas afterwards the inverse was observed. However, since the first 2 weeks account for 95% of rac-HCQ renal excretion, the total urinary excretion of S(+)-HCQ clearly surpassed that of R(-)-HCQ.(ABSTRACT TRUNCATED AT 250 WORDS)