The contribution of CYP1A2 to the metabolism of the dietary heterocyclic amines, 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) and 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) in vivo in humans, has been determined with furafylline, a highly selective inhibitor of this enzyme. The inhibitory potential of furafylline in vivo was first assessed by determining its effect on clearance of phenacetin to paracetamol by the model CYP1A2-dependent O-deethylation pathway. Furafylline inhibited this reaction by > 99% in all subjects, thus demonstrating its applicability to determining the contribution of CYP1A2 to a given reaction in vivo. A group of 6 healthy male volunteers received either placebo or 125 mg furafylline, in a double-blind balanced crossover design, 2 h prior to consuming a test meal of fried beef containing a known amount of amines. The excretion of PhIP and MeIQx in urine was determined during the subsequent 28 h, using gas chromatography-mass spectrometry. Following furafylline, the excretion of unchanged MeIQx increased 14.3-fold, while that of PhIP increased 4.1-fold (P < 0.01, paired t test). Elimination of both amines was first order and very rapid, with half-lives of < 5 h. The elimination rate constants did not change following furafylline, suggesting that total clearance is limited by hepatic blood flow. Because the elimination of the amines was first order, it was possible to calculate the contribution of CYP1A2 to the clearance of the amines. CYP1A2-catalyzed metabolism accounts for 91% of the elimination of ingested MeIQx and 70% of ingested PhIP, most likely via N-hydroxylation.