Explant cultures of both human and rat colon have the ability to enzymatically convert various classes of chemical carcinogens, such as polycyclic aromatic hydrocarbons, N-nitrosamines, mycotoxins and hydrazines, into metabolites which react with cellular macromolecules. The effect of various co- and anticarcinogens of colon carcinogenesis on the metabolism of benzo(a)pyrene (BP) and of 1,2-dimethylhydrazine (DMH) by the rat colon has been studied. Different segments of the rat colon metabolize BP and DMH to varying extents. The metabolism of BP, less in the colon was approximately 10% of the level found in the human bronchus; the major metabolites extractable with ethylacetate, were quinones, a peak containing 9,10-diol and 7/8,9-triol and 7,8-diol. The metabolite pattern of BP in human and rat colon were quite similar. Secondary bile acids significantly decreased the binding of DMH to rat colon, while enhancing the binding level of BP to DNA in human colon. Benz(a)anthracene did not enhance either activity of aryl hydrocarbon hydroxylase or BP binding to DNA in human and rat colon.