To develop a rapid HPLC method for the determination of cytochrome P-450 CYP1A2 activity.A 300-microL plasma was prepared by extraction with 5-mL chloroform/isopropanol (9:1), and beta-hydroxyletheophylline was added as internal standard (IS). Samples were separated on an ODS column by a gradient elution system, of which mobile phase consisted of 0.05% acetic acid, acetonitrile, and methanol. The compounds of interest were monitored at 282 nm by UV detector.No potential interfering peaks were found. Paraxanthine (17X), IS and caffeine (137X) were rapidly eluted with baseline resolution, and their retention time was less than 13 min. The detection limits of both 17X and 137X were 0.1 mumol.L-1. Linear relations ranged over 1-100 mumol.L-1 and 1-200 mumol.L-1 with correlation coefficient of 0.9999 and 0.9987, respectively, for 17X and 137X. The coefficients of variation were within 6% for 17X, and 10% for 137X. The average recoveries for both compounds were ranged from 96% to 108%.This method is sensitive and rapid, and can be used for population studies of CYP1A2.