The cardioactive drug quinidine has a narrow therapeutic index; consequently, determination of serum quinidine concentrations can be important. We describe a relatively rapid and specific assay for quinidine in serum by high-performance liquid chromatography. It is suitable for use in patient monitoring or pharmacodynamic studies. Alkalinized serum is extracted with benzene, which is evaporated under nitrogen and reconstituted with methanol; an aliquot is chromatographed. Quinidine is separated from its metabolites and dihydroquinidine (a contaminant in quinidine raw materials). The retention time for quinidine is 4 min 10 s, for dihydroquinidine it is 5.5 min. Results for patients' sera by this assay method and the double-extraction method of Cramer and Isaksson [Scand. J. Clin. Lab. Invest. 15, 553 (1963] correlate well (r = .975).