A new reverse-phase high-pressure liquid chromatography assay allowing simultaneous but separate quantitation of urinary levels of quinidine and its major metabolites, 2'-quinidinone, 3-OH-quinidine and a newly detected N-oxide, is described. The compounds were separated on a alkyl phenyl column using 0.05 M phosphate buffer pH 4.5/acetonitrile/tetrahydrofuran (80 : 15 : 5, v/v) as mobile phase and were detected by UV at lambda = 230 nm. The assay procedure includes extraction of the compounds from urine samples into a mixture of dichloromethane/isopropanol (4 : 1, v/v), evaporation of the organic extracts to dryness and reconstitution of the residue in acetonitrile. The new assay was compared to a modification of the Cramer and Isaksson fluorescence assay which has recently been recommended for analysis of quinidine in urine. The consistently higher quinidine levels observed in the fluorescence assay could be accounted for by the quinidine levels and metabolite carry-over as determined by HPLC.