A sensitive, accurate and specific spectrodensitometric method has been developed for the determination of quinidine in biological fluids. It involves extraction of quinidine, dihydroquinidine and metabolites, their separation on thin layers and quantitation of the corresponding spots by direct scanning in a densitometer at 278 nm. A linear relationship was obtained between the ratio of the peak area of an unknown sample to that of the standard and the concentration of the compounds at 0.4-4microgram/ml. The recovery from plasma was from 96 to 103% for quinidine and from 93.5 to 98.5% for dihydroquinidine. A comparison was made between this thin-layer chromatographic method and the fluoriemtric assay frequently used for the determination of quinidine in plasma at present. The method is recommended for clinical assays and pharmacokinetics studies.