A rapid, sensitive, accurate method for determination of quinidine in plasma has been developed using ion-pair extraction and high-performance liquid chromatography. The method, which is capable of distinguishing between quinidine and dihydroquinidine, involves acidification of plasma with perchloric acid, extraction with methyl isobutyl ketone and chromatography of the carbonate-washed extract on a silica gel column with a mobile phase of methylene chloride-hexane-methanol--perchloric acid (60:35:5.5:0.1) followed by fluorometric detection. The procedure is sensitive to below 50 ng/ml (coefficient of variation 6.6%) and compares favourably with a standard spectrofluorometric method when tested with plasma from volunteer subjects.