The nature of the cytochrome P-450-dependent enzyme reactions giving rise to four primary metabolites of caffeine was investigated using microsomes isolated from livers of human kidney donors. Metabolite formation proceeded at a lower rate than that predicted from in vivo caffeine elimination half-lives, as has been observed in other species using this compound as a substrate in microsomal incubations. Kinetic experiments indicated that the formation of each of the N-demethylated metabolites paraxanthine, theobromine and theophyline was mediated by both a high- and a low-affinity catalytic site over a substrate concentration range from 0.05 mM to 80.0 mM, although only the high-affinity component is likely to be of any importance at normally encountered in vivo caffeine concentrations. 7-Ethoxyresorufin and acetanilide, selective substrates for two polycyclic aromatic hydrocarbon (PAH)-inducible isozymes of cytochrome P-450 in the mouse (P1-450 and P3-450, respectively) were each able to inhibit competitively the formation of caffeine metabolites by human liver microsomes, while caffeine could in turn similarly inhibit the biotransformations of these two compounds. The isozyme-selective P-450 inhibitor alpha-naphthoflavone (ANF) potently inhibited the high-affinity component of caffeine N-demethylations, while 1-phenylimidazole (PI) was a more potent inhibitor of the low-affinity component. The inhibition studies also indicated that the formation of 1,3,7-trimethyluric acid was mediated by both ANF-sensitive and PI-sensitive sites. Taken together, the data support suggestions from in vivo studies that a PAH-inducible isozyme of cytochrome P-450 plays a significant role in the biotransformation of caffeine in man.