Abstract: A rapid HPLC procedure has been developed for the determination of procainamide (PA) and N-acetyl procainamide (NAPA) in serum. PA and NAPA were extracted from human serum by using a mixture of n-propanol and chloroform(1:9). The organic phase was evaporated to dryness under a gentle stream of dry nitrogen at 40～50℃. The residue was redissolved in 200 μl mobile phase containing procaine internal standard and 20μl samples were analysed on a column (3.9 mm × 15 cm) of, NOVA-PAK C 18 (51μm) by using the mobile phase (1ml/min) of acetic acid (40ml)-sodium acetate (4g)-H₂O (1000ml)-acetonitrile (100ml) and detecting at 254nm. The calibration plot was rectilinear over the range 0.5～121μg/mlof PA, and 0.5～0.6μg/ml of NAPA. The average recovery of PA and NAPA was 99.4% and 100.5%, while the average CV was 4.16% and 4.13%, respectively. The method was found to be adequate for monitoring the human serum concentration vs. time profile of PA and NAPA after oral administration of PA and for determining acetylator phenotype based on a ratio of NAPA to PA in serum.