To compare the inhibitory effectS of quinidine and quinine on liver microsome bufuralol 1Aydroxylase (BH). aryr hydrocarbon hydroxyl- ase (AHH). and 7-ethoxycoumarin O-deethylase (ED) activities in man and rat. METHODS: A normal phase HPLC and the fluorescence spectrometry were used to assay the enzyme activities. RESULTS: Both quinidine and quinine produced a concentration-dependent inhibition to liver microsome BH, AHH, and ED in man and rat. Thelr median inhibitory concentrations (ICsO) oii liver microscme BH, AHH. and ED low and high atfinjfy phases were 0.2. 378. 2952P and >5000 moI.L-1 for quinine in man. 290, 613 1465. 1595. UmoI.L-1 for quinidine in rat; 29, 207, 808, and >5000 umoI.L-l for quinine in man. and 31, 54, 597, and 2508 UmoI.L-1 for quinine in rat, respectively. CONCLUSlON: Quinidine is a species- and stereo-selective potent inhibitor to human liver microscme BH.