Previously published estimates of pharmacokinetic characteristics of quinidine can be shown to be dependent on whether the investigators have used analytical methods which are specific for quinidine. Areas under the plasma concentration-time curve and peak plasma concentrations after administration of the drug were higher and clearance values consequently lower in studies utilizing nonspecific assays unable to distinguish quinidine from its metabolities. The error introduced is larger after oral administration as a result of marked first-pass metabolism of quinidine. The absolute oral bioavailabilities from pharmaceutical preparations might therefore be estimated higher in studies with assays including metabolites in the determination. Although the pharmacodynamic response to quinidine is related to the plasma concentration, the therapeutic window of drug concentrations has been defined only using nonspecific assays. In light of the availability of newly developed specific assays, redefinition of the range of therapeutic plasma concentrations is opportune.