Using a hybrid ion-exchange reverse phase HPLC system, we found that F344 rat liver microsomes, in the presence of an NADPH-generating system, can metabolize methylazoxymethanol (MAM), a colon and liver carcinogen, to methanol and formic acid. This is in contrast to the spontaneous decomposition of MAM which yields methanol and formaldehyde. The metabolism of MAM by rat liver microsomes is sensitive to inhibition by carbon monoxide as well as to inhibition by 3-methylpyrazole (3-MeP) and 4-iodopyrazole (4-IP), with 4-IP being more potent in this respect than 3-MeP. Pretreatment of rats with 4-IP decreased the level of MAM acetate-induced DNA methylation in both the liver and the colon mucosa. In contrast, pretreatment with 3-MeP decreased MAM acetate-induced liver DNA methylation, but increased DNA methylation in the colon mucosa. This differential effect of the two compounds on DNA methylation in the liver and the colon suggests that different enzymes are responsible for activation of the carcinogen in the two organs.