It is suggested that a CYP- or peroxidase-mediated activation of Sudan I or a combination of both mechanisms as an explanation for the organ specificity of this carcinogen for liver and urinary bladder in animals. The Sudan I metabolites formed by peroxidase are much less likely to be formed at physiological conditions, because "in vivo" there are many nucleophilic molecules present which scavenge the Sudan I reactive species. Hence, formation of adducts of Sudan I reactive species with nucleophilic species, such as DNA, tRNA, proteins, polynucleotides, and polydeoxynucleotides seems to be the preferred reaction under physiological conditions, with deoxyguanosine as the major target for Sudan-I DNA binding, followed by deoxyadenosine.