Quinidine and quinine are stereoisomers. When given sequentially or simultaneously, the renal clearance of values of quinidine and quinine were measured in seven healthy volunteers and when given simultaneously, to an additional five elderly men. Analytic specificity was provided by an HPLC drug assay. Protein binding was measured by equilibrium dialysis. The quinidine clearance exceeded the quinine clearance in every individual. The mean (+/- SD) ratio of these clearances was 4.2 +/- 1.4 when the drugs were given together and 4.4 +/- 2.3 when they were given separately in the younger subjects and 3.4 +/- 0.5 when given simultaneously to the elderly. Calculating clearance of drug in serum water (unbound drug clearance) revealed a quinidine clearance of 6.1 +/- 2.3 times that of creatinine measured simultaneously, and for quinine it was 1.5 +/- 0.6 times that of creatinine. We conclude that there is stereoselective net renal tubular secretion of quinidine over quinine indicating stereoselectivity of this renal tubular transport process.