The low turnover of caffeine in vitro by human liver microsomes makes the study of the metabolic pathways of this compound difficult. Analytical methods with high sensitivity and specificity are needed for the detection of its metabolic products. A method based on the on-line radiometric determination of [8C-3H]caffeine and its principal metabolite (paraxanthine) in man has been developed using reversed-phase high-performance liquid chromatography. The method has been successfully employed in preliminary studies of the kinetics of this reaction.