Abstract: Caffeine was used as a metabolic probe to measure, in 120 healthy volunteers, the activities of three enzymes, deduced to be N-acetyltransferase(NAT2), CYP1A2 and xanthine oxidase(XO). The caffeine metabolites of 5-acetylamino-6-formylamino-3-methyluracil(AFMU), 1-methylxanthine(1X), 1-methyluric acid(1U), 1,7-dimethylxanthine(17X), and 1,7-dimethyluric acid(17U) in urine were determined with HPLC after 4～5 hours of caffeine drink. The ratios of AFMU/1X or AFMU/(AFMU+1X+1U), (AFMU+1X+1U)/17X or (AFMU+1X+1U)/17U, and 1U/1X or 1U/(1X+1U) were used as the index of NAT2, CYP1A2, and XO activities respectively. Frequency distribution analysis of the metabolic ratios of NAT2 indicated two distinct group with 20 slow acetylators and 100 rapid acetylators. Similar CYP1A2 activity was found in Chinese compared with European volunteers. Frequency analysis of CYP1A2 indicated the log normal distribution in 120 Chinese. The CYP1A2 index was much higher in smokers than that in non-smokers. But no obvious difference was observed between young and old volunteers. The XO index also showed log normal distribution and has the similar value compared with European volunteers.The concentration variations of 1X and 1U in young volunteers were much lower than that in old volunteers. 