A method is described for the measurement of theobromine, theophylline and caffeine in serum and saliva by high-performance liquid chromatography (HPLC). A chloroform/isopropanol extract (85:15, v/v) is evaporated to dryness and chromatographed on a 100 X 4.5 mm id Hypersil octadecylsilane column with UV detection at 280 nm. Theobromine, theophylline, caffeine and the internal standard proxyphylline are satisfactorily resolved with an elution system of acetonitrile/tetrahydrofuran/50 mM acetate buffer, pH 4.0, (4:1:95, v/v). No interference is observed from the presence of xanthine metabolites or any of a number of common drugs examined. A good correlation was observed between the concentrations of caffeine in serum and in saliva suggesting that salivary measurements may be useful for the study of caffeine pharmacokinetics in man. Caffeine levels determined by the HPLC procedure described here agreed well with those obtained by a radioimmunoassay method. The method is also suitable for determining the xanthine content of beverages by direct injection of diluted samples.