A sensitive high performance liquid chromatographic assay was developed for the determination of caffeine and its dimethylxanthine metabolites in human plasma. Caffeine, paraxanthine, theophylline, and theobromine were extracted from 0.5 mL of matrix with methylene chloride. Separation of the analytes was achieved using a mobile phase consisting of 0.1 M sodium acetate pH 4.5, methanol, tetrahydrofuran, and distilled water (10:6.5:1.4:82.1, v/v) and a Microsorb MV C18analytical column with UV detection at 274 nm. Excellent linearity was observed for each analyte over the concentration range of 20 – 8000 ng/mL.