It is hypothesized that differences in the carcinogenicity of certain aza-arenes and their carbocyclic analogs may originate from the ability of the former to form covalent hydrates. The hypothesis is primarily based on a comparison of the carcinogenicity of several aza-arenes with that of corresponding carbocyclic compounds and of aromatic amines which are structurally related to the hydrated aza-arenes. Additional evidence is provided by the regiochemistry of cytochrome P-450 catalyzed oxidation of quinoline.